fix: skip static checks for MR pipelines

GitLab way of running MR pipelines results in our jobs requiring
undefined dependencies (`phpcs` requires `prepare vendor prefixed`).
Everything works properly for regular branch and tag pipelines, but
merge requests are triggering error or invalid YAML.

Simply, disable those jobs in CI.

Signed-off-by: Bart Jaskulski <bjaskulski@protonmail.com>

gitlab-ci-1.2.yml

 variables:
-  WPDESK_CI_VERSION: "2023.11.17"
+  WPDESK_CI_VERSION: "2023.11.20"
   MYSQL_ROOT_PASSWORD: "mysql"
   MYSQL_DATABASE: "wptest"
   MYSQL_USER: "mysql"
@@ -43,3 +43,4 @@ include:
       - '/includes/deploy.yml'
       - '/includes/pages.yml'
       - '/includes/mixins/.after-codeception.yml'
+      - '/includes/mixins/.skip-mr.yml'

includes/mixins/.skip-mr.yml

+.skip-mr:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+      when: never

includes/tests/static-analysis.yml

@@ -3,6 +3,7 @@
   needs:
     - prepare prefixed vendor
   stage: tests
+  extends: .skip-mr
 
 # Run phpstan in strict mode (failing pipeline) only if we have baseline, otherwise allow to fail with a warning, suggesting to create one. This behavior will change at the end of the year, when any PHPStan failure will be considered fatal
 phpstan: