Test3: pin macOS runner to macos-15

Author: mzecc

.github/workflows/ci-main-only.yaml

@@ -13,7 +13,7 @@ jobs:
       matrix:
         # Not running on windows, see https://github.com/openscm/gcages/issues/36
         # os: [ "ubuntu-latest", "macos-latest", "windows-latest" ]
-        os: [ "ubuntu-latest", "macos-15" ]
+        os: [ "ubuntu-latest", "macos-15-arm64:20250928.2397" ]
         # Just test on a sensible version
         # Use 3.11 while cicero-scm uses distutils in openscm-runner,
         # see https://github.com/openscm/openscm-runner/blob/261bd177b86a708b0080eed245b6511ebb0051bb/src/openscm_runner/adapters/ciceroscm_adapter/ciceroscm_wrapper.py#L10
@@ -27,7 +27,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
           uv-dependency-install-flags: "--all-extras --group tests"
       - name: Set DYLD path
-        if: matrix.os == 'macos-15'
+        if: matrix.os == 'macos-15-arm64:20250928.2397'
         run: |
           # # Useful if the path appears to have changed
           # find / -name 'libgfortran.5.dylib' 2>/dev/null