Override CI_COMMIT_REF_SLUG for QA branches

The `gitlab:assets:compile` job isn't run for the QA branches, thus
there's no Docker image correspinding these branches in the registry.

By overriding `CI_COMMIT_REF_SLUG` to `master` for QA branches, the
`fetch-assets` job in the `omnibus-gitlab` pipeline will pull the
`master` assets Docker image.
Signed-off-by: Rémy Coutable <remy@rymai.me>

scripts/trigger-build

@@ -106,12 +106,18 @@ module Trigger
     def extra_variables
       {
         'GITLAB_VERSION' => ENV['CI_COMMIT_SHA'],
-        'GITLAB_REF_SLUG' => ENV['CI_COMMIT_REF_SLUG'],
+        'GITLAB_REF_SLUG' => ref_slug,
         'ALTERNATIVE_SOURCES' => 'true',
         'ee' => Trigger.ee? ? 'true' : 'false',
         'QA_BRANCH' => ENV['QA_BRANCH'] || 'master'
       }
     end
+
+    def ref_slug
+      return 'master' if ENV['CI_COMMIT_REF_SLUG'] =~ %r{(\Aqa[/-]|-qa\z)}
+
+      ENV['CI_COMMIT_REF_SLUG']
+    end
   end
 
   class CNG < Base