CI: only deploy the site when pushing (#382)

Not, for example, on scheduled runs

Author: ISSOtm

.github/workflows/build-and-deploy.yml

@@ -71,8 +71,8 @@ jobs:
   deploy:
     runs-on: ubuntu-latest
     needs: build
-    # Do not run this unless *pushing* (`heads`) to `master`
-    if: github.ref == 'refs/heads/master'
+    # Do not run this unless *pushing* to `master`
+    if: github.event_name == 'push' && github.ref == 'refs/heads/master'
     steps:
     - name: Checkout pandocs/gh-pages
       uses: actions/checkout@v2