When "build targets" are used, the sccache server isn't
being used/integrated properly with make.
By moving rusttests to a separate build "application",
`client.mk` is invoked normally, and sccache is happy.
* Move rusttests to a new build "application".
* Move and denormalize rusttests mozconfigs. The new rusttests
mozconfigs should expand to be mostly identical to their old
versions, except with `MOZ_AUTOMATION_*=0` and
`--enable-application=tools/rusttests`.
They will differ a bit from their old variants because some
of them originally didn't source from their
respective `$app_mozconfigs/common`.
* Modify `baseconfig.mk` to change tiers if running rusttests.
* Adjust CI config to use new "rusttests" build application.
Differential Revision: https://phabricator.services.mozilla.com/D106294