Bug
1874234 - don't run linux tests through `mach python`. r=ahal
Test tasks don't populate a repository, so if one exists it'll have been
set up by a previous task on the same worker; we don't want to rely on
it in that case, as that makes tasks unreproducible (and breaks,
currently, because mach python doesn't like python 3.7).
Explicitly use mach python for android generate-profile and
webrender-android-emulator tasks, which do set up a checkout and require
in-tree packages.
Differential Revision: https://phabricator.services.mozilla.com/
D198704