[Nix] Install comm cache in nix.conf

Summary:
Use the legacy way to define an alternative binary cache.
This avoids trying to rely on experimental flakeConfig feature which
seem to need some time to stabilize, see
https://github.com/nixos/nix/issues/6672 for more details.

https://linear.app/comm/issue/ENG-2126

Test Plan:

# On a non-NixOS machines (nix.conf is readonly)
cat /etc/nix/nix.conf
# confirm it's missing "https://comm.cachix.org" as a substituter

./scripts/install_nix.sh

# confirm "https://comm.cachix.org" is now a substituter

Reviewers: varun, atul

Reviewed By: atul

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5561