SMR-17: ensure correct ref is checked out and that buildx works

.github/workflows/e2e-agora.yaml

@@ -27,7 +27,12 @@ jobs:
           # merge commit in this context. The current solution is to checkout the PR HEAD instead
           # and enable the branch protection rule "Require branches to be up to date before
           # merging".
-          ref: ${{ github.event.pull_request.head.sha }}
+          ref: ${{ github.event_name == 'pull_request_target' && github.event.pull_request.head.repo.full_name != github.repository && github.event.pull_request.head.sha || github.ref }}
+
+      - name:
+          Switch from the detached HEAD of the merge commit to a new branch
+          # Buildx does not work on a detached HEAD
+        run: git switch -c new-branch
 
       - name: Derive appropriate SHAs for base and head for `nx affected` commands
         uses: nrwl/nx-set-shas@v4