ci: Only run glib-macros tests on stable

Some of the tests are checking compiler errors and they might change
between Rust versions.

Author: sdroege

.github/workflows/CI.yml

@@ -116,6 +116,7 @@ jobs:
         run: cargo build --manifest-path glib-macros/Cargo.toml
       - name: "glib-macros: test"
         run: cargo test --manifest-path glib-macros/Cargo.toml
+        if: matrix.rust == 'stable'
       # glib-macros in 2 levels dependent
       - name: Check two levels glib dependent
         run: cargo check