[Nix] Fix arc diff

Summary:
Noticed that arc diff was failing in new terminal sessions
due to sh: line 1: php: command not found. Looks like some arcanist commands
forks the process and some logic still calls php. This just adds php
to PATH.

Test Plan:

# somehow remove PHP from path? or just trust me :)
# This can be used from any branch
nix develop github:commE2E/comm?ref=jonringer/fix-arcanist

# use `arc diff` or another command which forks the process

Reviewers: varun, ashoat

Reviewed By: ashoat

Subscribers: tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4688