ci: use commit sha instead of head branch

Using the HEAD branch ref when checking for the api-tests workflow
can lead to checking the status of multiple runs and failing,
because we only want successfull ones.

.github/workflows/deploy-instance.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Wait for API tests
         uses: lewagon/[email protected]
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.sha }}
           check-name: 'api-test'
           repo-token: ${{ secrets.GITHUB_TOKEN }}