[Chore] Make shellcheck happy

Summary:
Shellcheck is failing on GitHub

Feel like I should have known this, as we use command -v in nixpkgs as well. Guess I just overlooked it because I wanted to follow the upstream patch as much as possible.

Test Plan: shellcheck CI

Reviewers: abosh, atul

Reviewed By: abosh, atul

Subscribers: ashoat, tomek, adrian, atul

Differential Revision: https://phab.comm.dev/D4819