add a do-not-submit workflow

Closes https://github.com/dart-lang/homebrew-dart/pull/143

GitOrigin-RevId: 1a826283717966e1d83abad4e959f5458d21d64d
Change-Id: Idc35a93c1249379e57c9975676f5a4e899c109dc
Reviewed-on: https://dart-review.googlesource.com/c/homebrew-dart/+/303402
Commit-Queue: Devon Carew <devoncarew@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
diff --git a/.github/workflows/gerrit-reviewed.yml b/.github/workflows/gerrit-reviewed.yml
new file mode 100644
index 0000000..07021a0
--- /dev/null
+++ b/.github/workflows/gerrit-reviewed.yml
@@ -0,0 +1,16 @@
+# A workflow to prevent PRs from being submitted through the GitHub UI.
+
+name: Do Not Submit
+
+on:
+  pull_request:
+    branches: [main]
+
+jobs:
+  do-not-submit:
+    name: Gerrit reviewed
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          echo "Do not submit this PR through github - this repo uses Gerrit for reviews."
+          exit 1