Fix release-verify-signatures workflow

Author: emlun

.github/workflows/release-verify-signatures.yml

@@ -17,6 +17,8 @@ jobs:
     steps:
     - name: check out code
       uses: actions/checkout@v3
+      with:
+        ref: ${{ github.ref }}
 
     - name: Set up JDK
       uses: actions/setup-java@v3