workflows: switch container build runners back to ubuntu-latest

It's now an alias for ubuntu-24.04, so we can stop hardcoding the OS
version.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

Author: bgilbert

.github/workflows/container-linux.yml

@@ -26,7 +26,7 @@ env:
 jobs:
   container:
     name: Build
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-latest
     steps:
       - name: Install dependencies
         run: |

.github/workflows/container-windows.yml

@@ -26,7 +26,7 @@ env:
 jobs:
   container:
     name: Build
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-latest
     steps:
       - name: Check out repository
         uses: actions/checkout@v4