Merge branch 'ebassi/ci-only' into 'master'

ci: Specify `only` for other jobs

See merge request GNOME/gtk!3052

.gitlab-ci.yml

@@ -43,6 +43,7 @@ style-check-diff:
     - .gitlab-ci/run-style-check-diff.sh
 
 .build-fedora-default:
+  extends: .only-default
   image: $FEDORA_IMAGE
   artifacts:
     when: always
@@ -121,6 +122,7 @@ installed-tests:
 
 
 .mingw-defaults:
+  extends: .only-default
   stage: build
   tags:
     - win32-ps
@@ -145,12 +147,12 @@ msys2-mingw64:
     CHERE_INVOKING: "yes"
 
 macos:
+  extends: .only-default
+  only:
+    - branches@GNOME/gtk
   stage: build
   tags:
     - macos
-  only:
-    - master
-    - merge_requests
   needs: []
   before_script:
     - bash .gitlab-ci/show-execution-environment.sh