🐛 Switch to Pull Request branch and not HEAD ref

Patch at 1631a18 doesn't actually work. Need to checkout the Pull Request branch instead of the HEAD ref. Fixes error like `fatal: You are not currently on a branch. To push the history leading to the current (detached HEAD)
state now, use ...`. Xref actions/checkout#331 (comment)

.github/workflows/conda-lock.yml

@@ -27,12 +27,16 @@ jobs:
           comment-id: ${{ github.event.comment.id }}
           reactions: eyes
 
-      # Checkout the pull request branch
-      - name: Checkout Repository
+      # Checkout the git repository
+      - name: Checkout repository
         uses: actions/checkout@v3
-        with:
-          repository: ${{ github.event.client_payload.pull_request.head.repo.full_name }}
-          ref: refs/pull/${{ github.event.issue.number }}/head
+
+      # Switch to pull request branch
+      # https://github.com/actions/checkout/issues/331#issuecomment-925405415
+      - name: Switch to Pull Request branch
+        run: hub pr checkout ${{ github.event.issue.number }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
       # Add an emoji reaction to comment to indicate that conda-lock is starting
       - name: Add reaction