Don't run nightly CI on tags

.github/workflows/ci.nightly.yml

@@ -17,6 +17,9 @@ concurrency:
 
 jobs:
   test-nightly:
+    # We do not run the full test suite on tags, because we already ran it on master before we made the release.
+    # We do build the docs on tags.
+    if: (github.event_name != 'push') || (github.ref_type == 'tag')
     timeout-minutes: 150
     runs-on: ${{ matrix.github-runner }}
     strategy: