[Nix] Add whitespace around message fixing glyph support

Summary:
Add more whitespace to help emphasize the message should be actionable.

https://linear.app/comm/issue/ENG-2254

Test Plan:

# Remove cache setting file
rm -f ~/Library/Caches/app.comm/enable-powerline

nix develop

# Answer yes to powerline prompt
# Assert message was emitted

Reviewers: varun, atul, abosh, O1 Nix

Reviewed By: atul, O1 Nix

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5699