gh-145098: Use macos-15-intel instead of unstable macos-26-intel (#148038)

Author: hugovk

.github/workflows/build.yml

@@ -206,16 +206,16 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # macos-26 is Apple Silicon, macos-26-intel is Intel.
-        # macos-26-intel only runs tests against the GIL-enabled CPython.
+        # macos-26 is Apple Silicon, macos-15-intel is Intel.
+        # macos-15-intel only runs tests against the GIL-enabled CPython.
         os:
         - macos-26
-        - macos-26-intel
+        - macos-15-intel
         free-threading:
         - false
         - true
         exclude:
-        - os: macos-26-intel
+        - os: macos-15-intel
           free-threading: true
     uses: ./.github/workflows/reusable-macos.yml
     with:

.github/workflows/reusable-macos.yml

@@ -52,15 +52,15 @@ jobs:
           --prefix=/opt/python-dev \
           --with-openssl="$(brew --prefix openssl@3.5)"
     - name: Build CPython
-      if : ${{ inputs.free-threading || inputs.os != 'macos-26-intel' }}
+      if : ${{ inputs.free-threading || inputs.os != 'macos-15-intel' }}
       run: gmake -j8
     - name: Build CPython for compiler warning check
-      if : ${{ !inputs.free-threading && inputs.os == 'macos-26-intel' }}
+      if : ${{ !inputs.free-threading && inputs.os == 'macos-15-intel' }}
       run: set -o pipefail; gmake -j8 --output-sync 2>&1 | tee compiler_output_macos.txt
     - name: Display build info
       run: make pythoninfo
     - name: Check compiler warnings
-      if : ${{ !inputs.free-threading && inputs.os == 'macos-26-intel' }}
+      if : ${{ !inputs.free-threading && inputs.os == 'macos-15-intel' }}
       run: >-
         python3 Tools/build/check_warnings.py
         --compiler-output-file-path=compiler_output_macos.txt