Add 3.4-asan builds, try 2

**What does this PR do?**

This PR is a second attempt at adding 3.4-asan builds (first attempt was
ruby#13); this version
is now atop ruby#14 .

It introduces a new "3.4-asan" build, based on the existing asan builds,
but just pointed at the `ruby_3_4` branch.

In ruby#13, we were building
the latest tagged 3.4 release, which I expect would be more stable
than just using `ruby_3_4` (and thus better for my downstream
purposes of "having a build that doesn't fail for non-asan-related
reasons").

Switching between both options is as simple as:

```diff
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 7eb72a8..d7608d9 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -29,6 +29,8 @@ jobs:
       with:
         repository: ruby/ruby
         path: ruby
+        fetch-tags: true
+        fetch-depth: 0 # Workaround for actions/checkout#1781
     - name: Set latest_commit
       id: latest_commit
       working-directory: ruby
@@ -37,7 +39,8 @@ jobs:
       id: latest_commit_3_4_asan
       working-directory: ruby
       run: |
-        git checkout ruby_3_4
+        LATEST_TAG=$(git tag --list | grep -E "v3_4_[0-9]+$" | sort -V | tail -n1)
+        git checkout "$LATEST_TAG"
         echo "commit=$(git rev-parse HEAD)" >> $GITHUB_OUTPUT
     - name: Check if latest commit already built
       uses: actions/github-script@v7
```

I personally prefer building from the tag, but happy to use the
branch option if that's preferrable.

**Motivation:**

The intention of "3.4-stable" is to provide the latest up-to-date
stable Ruby, so that we can reliably use it as a breaking CI step.

As discussed in ruby/setup-ruby#682, the
current ruby-asan builds are a bit of a "sharp edge" when used in CI
because they may break due to changes that are completely unrelated to
asan.

Building asan rubies is a bit awkward still, as e.g. ruby-build and
other version managers don't have support for it, and it requires
very modern versions of specific system tools (e.g. clang).

**Additional Notes:**

In particular, I decided to not touch the logic that determines weather
there's a more recent commit to build or not. This does mean that if
ruby master sees no commits, but there's changes in the 3.4 branch,
this won't be picked up immediately; and it also means that if there's
a new master commit and no change to the 3.4 branch we still rebuild
3.4-asan.

My thinking is that given that ruby#14 added caching already, this approach
keeps things simple.

Let me know if you're not convinced, and I can change that.

**How to test the change?**

I've built this in the downstream fork, and manually downloaded the resulting Ruby and it seems to be in good shape and with asan working fine.

* Successful run: FIXME
* Resulting builds: FIXME

.github/workflows/build.yml

@@ -19,6 +19,7 @@ jobs:
     outputs:
       should_build: ${{ steps.check_commit.outputs.should_build }}
       commit: ${{ steps.latest_commit.outputs.commit }}
+      commit_3_4_asan: ${{ steps.latest_commit_3_4_asan.outputs.commit }}
       previous_release: ${{ steps.check_commit.outputs.previous_release }}
       build_matrix: ${{ steps.matrix.outputs.build_matrix }}
       reuse_matrix: ${{ steps.matrix.outputs.reuse_matrix }}
@@ -32,19 +33,26 @@ jobs:
       id: latest_commit
       working-directory: ruby
       run: echo "commit=$(git rev-parse HEAD)" >> $GITHUB_OUTPUT
-
+    - name: Set latest commit (3.4-asan)
+      id: latest_commit_3_4_asan
+      working-directory: ruby
+      run: |
+        git checkout ruby_3_4
+        echo "commit=$(git rev-parse HEAD)" >> $GITHUB_OUTPUT
     - name: Check if latest commit already built
       uses: actions/github-script@v7
       id: check_commit
       with:
         script: |
           const latestDevCommit = "${{ steps.latest_commit.outputs.commit }}"
+          const latest34Asan = "${{ steps.latest_commit_3_4_asan.outputs.commit }}"
           const { owner, repo } = context.repo
           let { data: release } = await github.rest.repos.getLatestRelease({ owner, repo })
           const firstLine = release.body.split('\n')[0]
           const latestReleaseCommit = firstLine.split('@')[1]
           console.log(`Latest release commit: ${latestReleaseCommit}`)
           console.log(`Latest ruby commit: ${latestDevCommit}`)
+          console.log(`Latest 3.4-asan: ${latest34Asan}`)
           core.setOutput('should_build', latestReleaseCommit !== latestDevCommit)
           core.setOutput('previous_release', release.tag_name)
     - name: Compute build and reuse matrix
@@ -56,7 +64,7 @@ jobs:
           const skipSlow = "${{ github.event_name == 'workflow_dispatch' && github.event.inputs.skip_slow == 'true' }}"
           const buildMatrix = JSON.stringify(
             skipSlow === 'false' ?
-              { os: osList, name: ['head', 'debug'], include: [{ os: 'ubuntu-24.04', name: 'asan' }] } :
+              { os: osList, name: ['head', 'debug'], include: [{ os: 'ubuntu-24.04', name: 'asan' }, { os: 'ubuntu-24.04', name: '3.4-asan' }]] } :
               { os: osList, name: ['head'] }
           )
           core.setOutput('build_matrix', buildMatrix)
@@ -116,6 +124,13 @@ jobs:
       with:
         repository: ruby/ruby
         ref: ${{ needs.prepare.outputs.commit }}
+      if: matrix.name != '3.4-asan'
+    - name: Clone ruby (3.4-asan)
+      uses: actions/checkout@v4
+      with:
+        repository: ruby/ruby
+        ref: ${{ needs.prepare.outputs.commit_3_4_asan }}
+      if: matrix.name == '3.4-asan'
     - name: Clone ruby-dev-builder
       uses: actions/checkout@v4
       with:
@@ -191,7 +206,7 @@ jobs:
         # Make the test timeouts more generous too (ASAN is slower)
         echo "RUBY_TEST_TIMEOUT_SCALE=5" >> $GITHUB_ENV
         echo "SYNTAX_SUGGEST_TIMEOUT=600" >> $GITHUB_ENV
-      if: matrix.name == 'asan'
+      if: matrix.name == 'asan' || matrix.name == '3.4-asan'
 
     # Build
     - run: mkdir -p ~/.rubies