Correct the arch handling for gcc

.github/workflows/build.yaml

@@ -216,8 +216,10 @@ jobs:
       run: |
         # Qt Online Installer only provides x86-64 binaries (not arm64) prior to Qt 6.2.0.
         if [[ '${{ matrix.qt }}' =~ ^(5|6\.[01])\. ]]; then arch=x86_64
-        # GitHub's macos-15 runners only support arm64, and no longer support x86-64.
-        elif [[ '${{ matrix.os }} == 'macos-15' ]]; then arch=arm64
+        # GitHub runners' (homebrew'd) gcc's only support support x86-64 prior to macos-14, and only arm64 after.
+        # Note: if we attempt to use multiple archs, gcc will warn, but continue, resulting in later failures.
+        elif [[ '${{ matrix.cc }}' == gcc-* ]]; then
+          if [[ '${{ matrix.os }}' == 'macos-13' ]]; then arx='x86_64'; else arch='arm64'; fi
         # Otherwise, default to universal binaries, where possible.
         else arch='arm64;x86_64'; fi
         echo "CMAKE_OSX_ARCHITECTURES=${arch}" >> $GITHUB_ENV