[Nix] Only prompt Powerline when an interactive shell

Summary:
Buildkite CI was polling indefinitely when running nix develop.

Only ask for powerline prompt if it's an interactive shell.

https://linear.app/comm/issue/ENG-1684

Test Plan:

# Reset powerline preference
rm ~/Library/Caches/app.comm/enable-powerline

nix develop
# Should get prompted for Powerline

For buildkite, the below run should pass
https://buildkite.com/comm/blob-nix/builds/50#0182c7f4-63c8-407c-a17a-3852aa3ae46b

Reviewers: atul, abosh, O1 Nix

Reviewed By: abosh, O1 Nix

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D4902