ci: gate sync on successful test workflow run

.github/workflows/sync.yml

@@ -1,26 +1,33 @@
 ---
 name: sync
 
+# Triggered on successful completion of the `test` workflow on main, so a
+# merged PR's pre-commit run gates the ArgoCD sync. Manual dispatch is also
+# supported for re-syncing without a new commit.
 on:
-  push:
-    branches:
-      - main
+  workflow_run:
+    workflows: [test]
+    types: [completed]
+    branches: [main]
   workflow_dispatch:
 
 permissions:
   contents: read
 
 jobs:
   sync:
+    if: github.event_name == 'workflow_dispatch' || github.event.workflow_run.conclusion == 'success'
     # In-cluster runner; kubectl uses its auto-mounted SA token to talk to
     # the API directly. The tfroot-runner image has kubectl preinstalled.
     runs-on: arc-tf
 
     steps:
       - name: Sync ArgoCD bootstrap Applications
+        env:
+          REVISION: ${{ github.event.workflow_run.head_sha || github.sha }}
         run: |
           for app in bootstrap-secrets gitops-operators gitops-workloads; do
             kubectl -n argocd patch application "$app" \
               --type=merge \
-              -p '{"operation":{"initiatedBy":{"username":"github-actions"},"sync":{"revision":"${{ github.sha }}"}}}'
+              -p "{\"operation\":{\"initiatedBy\":{\"username\":\"github-actions\"},\"sync\":{\"revision\":\"${REVISION}\"}}}"
           done