Bug 1374940 adds a MOZ_TOOLCHAINS environment variable with a list of
path@task-id strings, where task-id is corresponding to the (possibly
optimized) toolchain job, and path corresponding to the
toolchain-artifact defined for that toolchain job.
We want to use that to pull artifacts instead of tooltool packages.
Hazard jobs use a specific tooltool-manifest field in their definition.
Since there is no post-processing happening on those definitions, and
since generalizing it would require adding the field to a bunch of
validation schemas, and the same code to various transforms, it's just
simpler to move to use environment variable definitions here too.
Eventually, tooltool manifests won't be necessary anyways, and those
environment variables will go away.
