Setup to build a GitHub PR without merging it

[fxlb]

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)