[Nix] Add rust-up to dev shell
Summary:
Add rust-up to dev shell.
Enables addition of toolchains for iOS and others
https://linear.app/comm/issue/ENG-1939
Test Plan:
nix develop which rustup # points to nix store
Reviewers: abosh, atul, varun, O1 Nix
Reviewed By: atul, O1 Nix
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D5305