fix(node): default-branch should use $default_branch (#1052)

Author: Benjamin E. Coe

synthtool/gcp/templates/node_library/.github/workflows/ci.yaml

@@ -1,7 +1,7 @@
 on:
   push:
     branches:
-      - default-branch
+      - $default-branch
   pull_request:
 name: ci
 jobs: