disable publish in action for pull requests

.github/workflows/unstable.yml

@@ -67,6 +67,7 @@ jobs:
           unstable/*
 
   publish:
+    if: github.ref == 'refs/heads/master' && github.event_name != 'pull_request'
     needs: test_and_build
     runs-on: ubuntu-latest
     steps:
@@ -115,4 +116,4 @@ jobs:
             repo: context.repo.repo,
             release_id: sid_release.data.id,
             body: "A persistent prerelease where build artifacts for the current tip will be deposited\n\n## Last updated: " + (new Date()).toDateString()
-          });
+          });