Only run the release-gate on workflow dispatch (#24366)

Author: zanieb

.github/workflows/release.yml

@@ -56,7 +56,7 @@ jobs:
   release-gate:
     # N.B. This name should not change, it is used for downstream checks.
     name: release-gate
-    if: ${{ inputs.tag != 'dry-run' }}
+    if: ${{ github.event_name == 'workflow_dispatch' && inputs.tag != 'dry-run' }}
     runs-on: ubuntu-latest
     # This environment requires a 2-factor approval, i.e., the workflow must be approved by another
     # team member. GitHub fires approval events on every job that deploys to an environment, so we