chore: CI: pin macos-11 to work around 12.7.1 breakage (#2946)

Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>

.github/workflows/ci.yml

@@ -134,7 +134,7 @@ jobs:
             # exclude seriously slow/problematic tests (laketests crash)
             CTEST_OPTIONS: -E 'interactivetest|leanpkgtest|laketest|benchtest'
           - name: macOS
-            os: macos-latest
+            os: macos-11
             release: true
             shell: bash -euxo pipefail {0}
             llvm-url: https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-apple-darwin.tar.zst
@@ -142,7 +142,7 @@ jobs:
             binary-check: otool -L
             tar: gtar  # https://github.com/actions/runner-images/issues/2619
           - name: macOS aarch64
-            os: macos-latest
+            os: macos-11
             release: true
             cross: true
             shell: bash -euxo pipefail {0}
@@ -225,7 +225,7 @@ jobs:
       - name: Install Brew Packages
         run: |
           brew install ccache tree zstd coreutils gmp
-        if: matrix.os == 'macos-latest'
+        if: matrix.os == 'macos-11'
       - name: Setup emsdk
         uses: mymindstorm/setup-emsdk@v12
         with: