Fix tf-plan: checkout PR head branch, not merge ref

Same fix as expand-terraform (PR #89). When expand pushes regenerated
files to the PR branch, tf-plan must checkout the updated branch HEAD,
not the stale merge ref captured at workflow trigger time.

Author: Alexanderamiri

.github/workflows/tf-plan.yml

@@ -51,7 +51,7 @@ jobs:
 
       - uses: actions/checkout@v6
         with:
-          ref: ${{ github.ref }}
+          ref: ${{ github.head_ref || github.ref }}
 
       - uses: hashicorp/setup-terraform@v4
         with: