ci: add workflow_dispatch trigger for manual CI runs

Allow manual triggering of CI workflow from GitHub Actions UI.
This enables running CI on bot-created PRs that don't automatically
trigger workflows due to GITHUB_TOKEN limitations.

Author: felixweinberger

.github/workflows/main.yml

@@ -3,6 +3,7 @@ on:
         branches:
             - main
     pull_request:
+    workflow_dispatch:
     release:
         types: [published]