Bug 1739067: Scope Mach virtualenv to be checkout-specific r=perftest-reviewers,ahal,sparky

Build and run the Mach virtualenv from a `state_dir` that is
"specific-to-topsrcdir".

As part of this, move `get_state_dir()` to `mach` so that it's usable
before `sys.path` entries are fully set up.

Differential Revision: https://phabricator.services.mozilla.com/D130383