Setup to build a GitHub PR without merging it

Currently the GitHub PR are built after a merge with:
```
git fetch -q origin +refs/pull/<PR_NUMBER>/merge:
git checkout -qf FETCH_HEAD
```
If there a setup flag to NOT merge the PR, but build the unmodified PR (Similar to the contributor's working directory)?
(Perhaps with: `git fetch -q origin +refs/pull/<PR_NUMBER>/head`)
