[Nix] Remove /usr/bin/ from PATH

Summary:
With https://phab.comm.dev/D5076, pod-patch was updated to not
require BSD sed. This removes the workaround put in place to make that previous
workflow happy.

Depends on D5076

Test Plan:

arc patch <this revision>
nix develop

# clean npm, and attempt pod install with new `pod-patch`
yarn cleaninstall
cd native/ios
pod cache clean --all
pod install

open -na xcode
# Comm project should build

Reviewers: varun, atul, max, abosh

Reviewed By: max

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5077