CI: rename Meson Homebrew/x86-64 cross file, for clarity

Also enforce the path to x86-64 GCC.

.gitlab-ci.yml

@@ -46,7 +46,7 @@ build_macos_x86_64:
   tags:
     - macOS
   script:
-    - arch -x86_64 meson setup -D buildtype=release --native-file scripts/homebrew-native.ini build
+    - arch -x86_64 meson setup -D buildtype=release --native-file scripts/homebrew-native-x86_64.ini build
     - arch -x86_64 meson compile -C build -v
   artifacts:
     paths:

scripts/homebrew-native.ini → scripts/homebrew-native-x86_64.ini

-# Meson native file for compiling under Homebrew
+# Meson native file for compiling under Homebrew / x86-64
 
 [binaries]
-cpp = 'g++-13'
+cpp = '/usr/local/bin/g++-13'
 flex = '/usr/local/opt/flex/bin/flex'
 bison = '/usr/local/opt/bison/bin/bison'