[Nix] Add node_modules/.bin to path

Summary:
Put node_modules bin directory on PATH as part of nix develop

https://linear.app/comm/issue/ENG-2253

Test Plan:

nix develop

which react-native # points to node_modules
which flow # points to node_modules

cd lib
flow

Reviewers: atul, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5651