Bug 1594520 - Change directory before running mach in .vscode/tasks.json on Windows. r=jya

The build is quite fragile if the path to mach is not precisely
formatted. Work around this by cd-ing to the appropriate directory and
then calling 'mach' with a relative path. This mimics a normal user
workflow in mozilla-build.

Differential Revision: https://phabricator.services.mozilla.com/D52968

--HG--
extra : moz-landing-system : lando