Do not create release for pull requests

Author: quantum5

.github/workflows/build.yml

@@ -33,6 +33,7 @@ jobs:
           path: python*.tar.gz
   release:
     runs-on: ubuntu-latest
+    if: github.event_name == 'push' && github.ref == 'refs/heads/master'
     needs: build
     steps:
       - name: Download artifacts