[direnv] Redirect use flake to /dev/null if COMM_NIX_LOGGING_LEVEL is "none"

Summary: Found the logging to be kind of annoying. Would prefer if Nix/Direnv worked completely in the background. Realize this is a personal preference so this change won't affect anyone unless they explicitly set COMM_NIX_LOGGING_LEVEL="none".

Test Plan:
Before setting COMM_NIX_LOGGING_LEVEL:

After setting COMM_NIX_LOGGING_LEVEL="none":

Reviewers: jon, ginsu, bartek, varun

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7052