Bug 1467605 - Disable processing of fetch dependencies; r=glandium a=tomprince

90dca0906337 accidentally broke mach artifact toolchain --from-build
because that code is attempting to load toolchain tasks in isolation.
The new "use_fetches" transform added to toolchain tasks requires
that "fetch" tasks are already processed and their references are
available to toolchain tasks.

This commit adds a mechanism to effectively disable the "use_fetches"
transform when called by mach artifact toolchain. It is a hack. I
suspect future planned work around artifacts/fetches will necessitate
additional changes to the mach artifact toolchain code. But this
can be deferred to a later day: this commit unbusts `mach artifact
toolchain` and isn't super hacky, so it seems more reasonable than
backing out fetch tasks completely.

Differential Revision: https://phabricator.services.mozilla.com/D1588