config: add push trigger to register workflow with GitHub Actions

Temporary push trigger on the feature branch so GitHub registers the
workflow for workflow_dispatch. Will be removed after triggering the run.

Author: cowwoc

.github/workflows/build-git-filter-repo.yml

@@ -11,6 +11,11 @@ on:
         description: 'git-filter-repo version tag (e.g., v2.38.0)'
         required: true
         default: 'v2.38.0'
+  push:
+    branches:
+      - 2.1-build-git-filter-repo-ci
+    paths:
+      - '.github/workflows/build-git-filter-repo.yml'
 
 jobs:
   build: