[nix] Run set-up-authoritative-keyserver.sh on nix startup

Summary:
issue: ENG-6845
We want to run set-up-authoritative-keyserver.sh when nix is run.

Test Plan: Ran nix develop and checked that the script is executed

Reviewers: michal, tomek, O1 Nix

Reviewed By: michal

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11310