ci: sync the mirror on release branch pushes too (Gitea's vmtest.yml runs on them)

.github/workflows/git-upriser-sync.yml

@@ -1,10 +1,10 @@
 name: Sync git.upriser.nl mirror
 
 # Trigger the Gitea pull-mirror immediately instead of waiting for its interval,
-# so Gitea builds/releases the ISO right after main or a tag lands on GitHub.
+# so Gitea builds/tests right after main, a release branch or a tag lands on GitHub (the mirror has no sync interval of its own).
 on:
   push:
-    branches: [main]
+    branches: [main, 'release/**']
     tags: ['*']
   workflow_dispatch: