Add the checkout ref for incorporating PR (#455)

Since we moved to using pull_request_target which executes the
workflow in the context of the base repository but the default HEAD
that gets checked out is now the main branch rather than the PR.
This ensures the pull request HEAD is checked out before running the
workflow.

.github/workflows/build-dev.yaml

@@ -13,6 +13,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          ref: "${{ github.event.pull_request.merge_commit_sha }}"
 
       - uses: actions/cache@v3
         with:

.github/workflows/e2e.yaml

@@ -16,6 +16,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          ref: "${{ github.event.pull_request.merge_commit_sha }}"
 
       # Install nix using cachix/install-nix-action if running on ARC runners
       # See: https://github.com/DeterminateSystems/nix-installer-action/issues/68

.github/workflows/synopsys.yaml

@@ -13,6 +13,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          ref: "${{ github.event.pull_request.merge_commit_sha }}"
 
       - name: Install devbox
         run: curl -fsSL https://get.jetpack.io/devbox | bash -s -- -f