github: add workflow-dispatch trigger for build workflow

Add a workflow-dispatch trigger to the build-workflow to enable
triggering build-tests on branches not covered from the match
pattern.

Signed-off-by: David Bauer <mail@david-bauer.net>

.github/workflows/build-gluon.yml

@@ -7,6 +7,7 @@ on:
       - 'v20[2-9][0-9].[0-9].x'
   pull_request:
     types: [opened, synchronize, reopened]
+  workflow_dispatch:
 
 permissions:
   contents: read