github: Only run the checklist checker for PRs against main

Currently this check is run on every push to stable branches which all
fail since they fail to fetch the non-existent .github/path-rules.txt
from the just-updated branches.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D60709

.github/workflows/checklist.yml

@@ -5,6 +5,7 @@ name: Checklist
 
 on:
   pull_request_target:
+    branches: [ main ]
     types: [ opened, reopened, edited, synchronize ]
 
 permissions: