ci: add bash syntax check

.gitea/workflows/ci.yaml

@@ -22,3 +22,12 @@ jobs:
           provenance: false
           tags: |
             backupsidecar:ci-test
+
+  check-syntax:
+    name: Check syntax
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - name: Check syntax
+        run: |
+          bash -n src/backup.sh