[Nix] Ensure cargo bin is on PATH

Summary:
Unless a user has their shell configured to add the subdirectory to
their path, then applications installed through cargo such as cargo-lipo will
not be found.

https://linear.app/comm/issue/ENG-2359

Test Plan:

nix develop

echo $PATH | grep .cargo/bin > /dev/null && echo "Cargo bin present"

Reviewers: atul, varun, O1 Nix, O4 Rust

Reviewed By: atul, varun, O1 Nix, O4 Rust

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5740