Bugfix: mamba used with an environment file never starts installation due to deadlock

[akihironitta]

Description of the Bug

When using mamba with an environment file, creating an environment never starts.

Here's a brief explanation of what's happening in mamba.py:

import asv.util as util
util.new_multiprocessing_lock("mamba_lock")
def _mamba_lock():
    return util.get_multiprocessing_lock("mamba_lock")
def run_mamba():
    with _mamba_lock():
        print("Hello, world!")
with _mamba_lock():
    run_mamba()

Description of the PR

This PR removes one of the locks around _run_mamba to avoid grabbing the lock inside the lock.

[HaoZeke]

LGTM, thanks @akihironitta.