gitlab: skip cirrus jobs on master and stable branches

On the primary QEMU repository we want the CI jobs to run on the staging
branch as a gating CI test.

Cirrus CI has very limited job concurrency, so if there are too many
jobs triggered they'll queue up and hit the GitLab CI job timeout before
they complete on Cirrus.

If we let Cirrus jobs run again on the master branch immediately after
merging from staging, that just increases the chances jobs will get
queued and subsequently timeout.

The same applies for merges to the stable branches.

User forks meanwhile should be allowed to run Cirrus CI jobs freely.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20211116112757.1909176-1-berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.gitlab-ci.d/cirrus.yml

@@ -40,6 +40,9 @@
     - cat .gitlab-ci.d/cirrus/$NAME.yml
     - cirrus-run -v --show-build-log always .gitlab-ci.d/cirrus/$NAME.yml
   rules:
+    # Allow on 'staging' branch and 'stable-X.Y-staging' branches only
+    - if: '$CI_PROJECT_NAMESPACE == "qemu-project" && $CI_COMMIT_BRANCH !~ /staging/'
+      when: never
     - if: "$CIRRUS_GITHUB_REPO && $CIRRUS_API_TOKEN"
 
 x64-freebsd-12-build: