[Nix] Pin node to 16.13.0

Summary:
Pin node to 16.13.0 to avoid import changes

https://linear.app/comm/issue/ENG-1391

Test Plan:

nix develop
cd keyserver
yarn build-prod && yarn prod

Reviewers: atul, varun

Reviewed By: atul

Subscribers: ashoat, palys-swm, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4542