Revert "chore(cov): drop ... .disabled file edit" (partial)

restore the conan_deps_cxxflags addition to xahau-ga-macos.yml.disabled.
prior commit framed it as dead code, but the .disabled rename is itself
a TEMP measure being reverted before merge - the field is needed for the
macOS workflow to keep building grpc 1.50.1 once it's re-enabled, since
the unconditional workaround was removed from the dependencies action.

.github/workflows/xahau-ga-macos.yml.disabled

@@ -107,6 +107,9 @@ jobs:
           compiler: apple-clang
           compiler_version: ${{ steps.detect-compiler.outputs.compiler_version }}
           stdlib: libcxx
+          # grpc 1.50.1 trips this on newer Apple Clang (clang-19+ frontend).
+          # Drop when grpc is bumped past the fix.
+          conan_deps_cxxflags: '["-Wno-missing-template-arg-list-after-template-kw"]'
 
       - name: Build
         uses: ./.github/actions/xahau-ga-build