[Nix] Warn user of writes to /etc/nix/nix.conf

Summary:
If a user already has nix installed, it's a bit
jarring to being prompted immediately for sudo access.

Indicate to the user what's going on to make experience
less frightening.

https://linear.app/comm/issue/ENG-1668

Test Plan:

sudo vim /etc/nix/nix.conf
# remove line with cores, max-jobs, or trusted-users

./scripts/install_nix.sh
# get prompted with message with missing value
# enter password

# assert value is now appended to /etc/nix/nix.conf
cat /etc/nix/nix.conf

Reviewers: abosh, atul, varun

Reviewed By: abosh

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D4882