Use the branch of the pull request when /bindings (#136)

.github/workflows/test.yml

@@ -86,6 +86,16 @@ jobs:
     steps:
 
       - uses: actions/checkout@v3
+
+      # When invoked by an issue comment event, the GitHub Actions runner runs
+      # on the default branch, so we need to switch the branch of the pull
+      # request. Since the branch name is not easily accessible via variables
+      # provided GitHub Actions, we use r-lib/actions, which is well-maintained.
+      - name: Switch branch (/bindings command)
+        if: github.event_name == 'issue_comment'
+        uses: r-lib/actions/pr-fetch@v2
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Set up R
         uses: r-lib/actions/setup-r@v2