[Nix] Add nixos cache as default for install_nix.sh

Summary:
https://phab.comm.dev/D5563#164976

This is to enable older versions of nix which don't
add cache.nixos.org as a substituter by default.

Also fix the sed find logic.

Test Plan:

cat /etc/nix/nix.conf
# confirm it's missing "https://cache.nixos.org" as a substituter

./scripts/install_nix.sh

# confirm "https://cache.nixos.org" is now a substituter

Reviewers: atul, varun

Reviewed By: atul

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5600