The remaining uses all need adjustements to in-tree mozconfigs, so they
all need to be done at once.

However, to make things slightly more intelligible, we do this in two
steps. This is step 1: we modify the use_toolchain transform to take care of
the transformation, while keeping the task definitions intact, so that
we only deal with mozconfig and build script adjustements here.

Differential Revision: https://phabricator.services.mozilla.com/D41890