Release only from merged commits on main branch. (#20)

.github/workflows/release.yml

@@ -1,9 +1,7 @@
 name: release
 
 on:
-  pull_request:
-    types:
-      - closed
+  push:
     branches:
       - main
     paths:
@@ -15,9 +13,8 @@ env:
 
 jobs:
   if_release:
-    if: |
-        ${{ github.event.pull_request.merged == true }}
-        && ${{ contains(github.event.pull_request.labels.*.name, 'release') }}
+    # Disallow publishing from branches that aren't `main`.
+    if: github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3