chore: CI: fix workflow change breaking unrebased PRs (#4441)

.github/workflows/ci.yml

@@ -280,12 +280,6 @@ jobs:
       CXX: c++
       MACOSX_DEPLOYMENT_TARGET: 10.15
     steps:
-      - name: Checkout
-        uses: actions/checkout@v3
-        with:
-          submodules: true
-          # the default is to use a virtual merge commit between the PR and master: just use the PR
-          ref: ${{ github.event.pull_request.head.sha }}
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@main
         if: runner.os == 'Linux' && !matrix.cmultilib
@@ -311,6 +305,18 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y gcc-multilib g++-multilib ccache
         if: matrix.cmultilib
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          # the default is to use a virtual merge commit between the PR and master: just use the PR
+          ref: ${{ github.event.pull_request.head.sha }}
+      # Do check out some CI-relevant files from virtual merge commit to accommodate CI changes on
+      # master (as the workflow files themselves are always taken from the merge)
+      - name: CI Merge Checkout
+        run: |
+          git fetch origin ${{ github.event.pull_request.merge_commit_sha }}
+          git checkout FETCH_HEAD flake.nix flake.lock
+        if: github.event_name == 'pull_request'
       - name: Cache
         uses: actions/cache@v3
         with: