Write ARCANIST into the environment while running arc

Summary:
Addresses a request to let Git commit hooks react differently when run via `arc`. While executing an `arc` command, write ARCANIST into the environment.

(I wrote the actual command as the value since it seemed like it could plausibly be more useful than `true` or `1`.)

Test Plan: Added `pre-commit` to `.git/hooks` which did `echo $ARCANIST`, verified the envvar was accessible when running via arc.

Reviewers: btrahan, frgtn

Reviewed By: frgtn

CC: aran

Differential Revision: https://secure.phabricator.com/D7281