chore: skip staging deploy for Renovate PRs (#124)

Dependency update PRs from Renovate don't need to be deployed to
staging. Uses job-level condition to check head branch prefix.

Reviewed-on: graham/towerops-web#124

.forgejo/workflows/staging.yaml

@@ -15,6 +15,7 @@ concurrency:
 jobs:
   deploy-staging:
     name: Deploy to Dokku Staging
+    if: ${{ !startsWith(github.head_ref, 'renovate/') }}
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout code