checkout generator repo on feature branch for testing

Author: Benjosh95

.github/workflows/sdk-pr.yaml

@@ -105,7 +105,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: "stackitcloud/stackit-sdk-generator"
-          ref: "main"
+          ref: "bf/add-javasdk-to-pipeline"
       - name: Build
         uses: ./.github/actions/build/java
         with: