[nix] Opt-out out of nvm usage with nix

Summary:
Nix is able to determine the desired version
when entering the development shell.

https://linear.app/comm/issue/ENG-908

Test Plan: nix develop, then do dev workflow in web and keyserver directories

Reviewers: ashoat, varun, atul, palys-swm

Reviewed By: ashoat, palys-swm

Subscribers: palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4421