Add workflow dispatch trigger to rebuild Action

We can't use the "Rebuild" label on a PR with conflicts because the
`pull_request` event runs on a synthetic merge commit.

Author: henrymercer

.github/workflows/rebuild.yml

@@ -3,6 +3,7 @@ name: Rebuild Action
 on:
   pull_request:
     types: [labeled]
+  workflow_dispatch:
 
 jobs:
   rebuild: