[Nix] Use file to determine user preference

Summary:
Avoid looking at PS1 to determine user's preference,
and just explicity ask them.

Use a file to save the preference between invocations (e.g. nix develop)

Test Plan:

nix develop
# should get prompted
# if yes, prompt should change
# if no, prompt should not change

rm ~/Library/Caches/app.comm/enable-powerline
exit

nix develop
# try other option

Reviewers: atul, varun, abosh

Reviewed By: abosh

Subscribers: ashoat, tomek, adrian

Differential Revision: https://phab.comm.dev/D4843