git-arc: Fix failure to call arc() function due to env(1)

[jrm]

Summary

As of b3e53f9fff11, git-arc attempted to call an internal shell function, arc(),
prefixed with env(1). However, because env does not execute shell functions,
the defined arc() function was never called, leading to errors.

% git arc create -r xxx HEAD
env: arc: No such file or directory
git-arc: could not create Phabricator diff

This change removes the unnecessary call to env(1), so the arc() function is
called.

Fixes: b3e53f9fff11 ("git-arc: Don't require devel/arcanist")

[jrm]

Accidentally pushed the wrong change.