Fix GHA skippable syntax (#13778)

Signed-off-by: Mathieu Velten <mathieuv@matrix.org>

.github/workflows/tests.yml

@@ -486,6 +486,7 @@ jobs:
           needs: ${{ toJSON(needs) }}
 
           # The newsfile lint may be skipped on non PR builds
-          skippable:
+          # Cargo test is skipped if there is no changes on Rust code
+          skippable: |
             lint-newsfile
             cargo-test