Consolidate Mach virtualenv management to the front of the Mach process. This obsoletes `./mach create-mach-environment` and simplifies the `sh` portion of the top-level `./mach` script. This helps ensure that the Mach virtualenv doesn't become out-of-sync and simplifies the mental model of the Mach virtualenv situation. Differential Revision: https://phabricator.services.mozilla.com/D120401