[Nix] Minimize arc's shebang length
Summary:
Fix darwin build of this package
Was about to send up another diff from the m1, and noticed that
master was failing to do nix develop after a rebase.
Test Plan:
Then run, from any m1 or intel mac, on any branch:
nix develop github:commE2E/comm?ref=jonringer/unpatch-php-arc
Reviewers: ashoat, varun
Reviewed By: ashoat
Subscribers: tomek, adrian, atul, abosh
Differential Revision: https://phab.comm.dev/D4727