gitlab-ci: make detached MR pipeline for external contributor's pipelines to run

The permissions for running CI will be restricted to external
contributors. It will only work for projects that use "detached MR
pipelines" ([1]).

Note that for it to actually work, a member with permission might have
to go to the "pipeline" tab of the merge request and click "run
pipeline". But this snippet is necessary for that.

[1] https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html

https://gitlab.freedesktop.org/freedesktop/freedesktop/-/issues/540#what-it-means-for-me-a-maintainer-of-a-project-part-of-gitlabfreedesktoporg

.gitlab-ci.yml

@@ -57,11 +57,11 @@ variables:
   #
   # This is done by running `ci-fairy generate-template` and possibly bumping
   # ".default_tag".
-  ALPINE_TAG:  'tag-c6ac275362b2'
-  CENTOS_TAG:  'tag-80e000f258af'
-  DEBIAN_TAG:  'tag-f961edd7a574'
-  FEDORA_TAG:  'tag-80e000f258af'
-  UBUNTU_TAG:  'tag-f961edd7a574'
+  ALPINE_TAG:  'tag-20ccfc6f0a87'
+  CENTOS_TAG:  'tag-cdc72bd04206'
+  DEBIAN_TAG:  'tag-bc68fd50d074'
+  FEDORA_TAG:  'tag-cdc72bd04206'
+  UBUNTU_TAG:  'tag-bc68fd50d074'
 
   ALPINE_EXEC: 'bash .gitlab-ci/alpine-install.sh'
   CENTOS_EXEC: 'bash .gitlab-ci/fedora-install.sh'
@@ -1117,4 +1117,11 @@ triage:issues:
     - gem install gitlab-triage
     - gitlab-triage -d --token $API_TOKEN --source-id $SOURCE_ID
   only:
-    - schedules
+    - schedules
+
+# Have detached MR pipeline (https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html)
+# https://gitlab.freedesktop.org/freedesktop/freedesktop/-/issues/540#what-it-means-for-me-a-maintainer-of-a-project-part-of-gitlabfreedesktoporg
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PIPELINE_SOURCE == 'push'

.gitlab-ci/ci.template

@@ -286,3 +286,10 @@ triage:issues:
     - gitlab-triage -d --token $API_TOKEN --source-id $SOURCE_ID
   only:
     - schedules
+
+# Have detached MR pipeline (https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html)
+# https://gitlab.freedesktop.org/freedesktop/freedesktop/-/issues/540#what-it-means-for-me-a-maintainer-of-a-project-part-of-gitlabfreedesktoporg
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PIPELINE_SOURCE == 'push'