ci: Set some sensible defaults

* Allow for all jobs to be cancelled if a newer commit is pushed
* Automatically retry jobs if they fail due to some infrastructure issue

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/144>

.gitlab-ci.yml

@@ -8,6 +8,18 @@ stages:
  - review
  - build
 
+default:
+    # Cancel jobs if newer commits are pushed to the branch
+    interruptible: true
+    # Auto-retry jobs in case of infra failures
+    retry:
+        max: 1
+        when:
+            - 'runner_system_failure'
+            - 'stuck_or_timeout_failure'
+            - 'scheduler_failure'
+            - 'api_failure'
+
 variables:
  LINT_LOG: "eslint-report.xml"
  JS_LOG: "js-report.txt"