os-extension-test: fix checkout ref (#210)

The tests are now running against the PR code and not main branch.

Co-authored-by: Alejandro Alvarez <avazquez@liquibase.com>

.github/workflows/os-extension-test.yml

@@ -154,6 +154,8 @@ jobs:
     needs: build
     steps:
       - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha || github.ref }}
 
       - name: Set up JDK ${{ matrix.java }}
         uses: actions/setup-java@v4