Use macos-15 for style check (#1249)

#1216 updated the test runner
image from `macos-12` to `macos-15`, but I forgot to update the image
for style checks. This PR updates the runner for style checks as well.

Author: qinsoon

.github/workflows/minimal-tests-core.yml

@@ -87,7 +87,7 @@ jobs:
         target:
           - { os: ubuntu-22.04, triple: x86_64-unknown-linux-gnu }
           - { os: ubuntu-22.04, triple: i686-unknown-linux-gnu }
-          - { os: macos-12, triple: x86_64-apple-darwin }
+          - { os: macos-15, triple: x86_64-apple-darwin }
         rust: ${{ fromJson(needs.setup-test-matrix.outputs.rust )}}
 
     name: style-check/${{ matrix.target.triple }}/${{ matrix.rust }}