Set required Xcode version in CI for gcc-13

The pre-installed gcc 13.3 (brew package 'gcc-13') only seems to work
with an older MacOSX.sdk version than the default in a macos-12 runner.
By setting a specific version gcc will use matching headers like:
/Applications/Xcode_14.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/Availability.h

Add a check if the compiler is already installed, which silences a warning.

.github/workflows/weekly.yml

@@ -155,7 +155,11 @@ jobs:
       - uses: actions/checkout@a5ac7e51b41094c92402da3b24376905380afc29 # v4.1.6
       - name: Prepare
         run: |
-          brew install gcc@${{ matrix.gcc-version }}
+          # Install gcc if not already available.
+          brew list gcc@${{ matrix.gcc-version }} &>/dev/null || \
+            brew install gcc@${{ matrix.gcc-version }}
+          # Switch to a working Xcode version.
+          sudo xcode-select -s /Applications/Xcode_14.0.app/Contents/Developer
       - name: Build and run tests
         env:
           CC: gcc-${{ matrix.gcc-version }}