pin Python version to 3.11 on Windows and Mac

.github/workflows/Windows-CI.yml

@@ -49,6 +49,11 @@ jobs:
             core.exportVariable('ACTIONS_CACHE_URL', process.env.ACTIONS_CACHE_URL || '');
             core.exportVariable('ACTIONS_RUNTIME_TOKEN', process.env.ACTIONS_RUNTIME_TOKEN || '');
 
+      - name: Set up Python
+        uses: actions/setup-python@65d7f2d534ac1bc67fcd62888c5f4f3d2cb2b236 #v4.7.1
+        with:
+          python-version: '3.11'
+
         # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - name: checkout
         uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 #v4.1.1
@@ -58,9 +63,9 @@ jobs:
         shell: bash
 
       - name: install-cmake
-        uses: lukka/get-cmake@359fbae4b163fa01633e6de228fa7f2a31ab1fc7 #v3.26.1
+        uses: lukka/get-cmake@4865386b66955d11be0abf8c112d0230023e742a #v3.27.9
         with:
-          cmakeVersion: 3.26.1
+          cmakeVersion: 3.27.9
           ninjaVersion: 1.11.1
 
       #      # Restore vcpkg from the GitHub Action cache service. Note that packages are restored by vcpkg's binary caching

.github/workflows/macos-ci.yml

@@ -45,6 +45,14 @@ jobs:
       # - libpng
       # - libvorbis
       # - python
+      #
+      # Note, though, that the Python version "varies between runners and can be changed unexpectedly",
+      # according to https://github.com/actions/setup-python#basic-usage . Hence the need to use setup-python.
+      - name: Set up Python
+        uses: actions/setup-python@65d7f2d534ac1bc67fcd62888c5f4f3d2cb2b236 #v4.7.1
+        with:
+          python-version: '3.11'
+
       - name: Install dependencies using homebrew
         run: brew install boost-python3 gtk+3 gtkglext sdl
 
@@ -74,7 +82,7 @@ jobs:
       # Ensure PRs are built against the PR Head as opposed to the merge commit
       - name: Conditionally relocate to PR HEAD
         if: github.event.pull_request
-        run: git checkout HEAD^2
+        run: git checkout ${{ github.event.pull_request.head.sha }}
 
       - name: Build it
         env: