[Nix] Expose rustc to development shell

Summary:
Add rustc to PATH on nix develop shell.

This isn't necessary when invoking cargo, as nixpkgs' cargo uses a wrapper
script to force rustc on PATH before invoking the cargo command. However, this
can be useful if someone would like to invoke rustc directly within the shell.

https://linear.app/comm/issue/ENG-1667

Test Plan:

nix develop
which rustc # points to nix store

Reviewers: ashoat, abosh, varun, O1 Nix

Reviewed By: ashoat, abosh, varun, O1 Nix

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D4872