Collect common options used in artifact build tests in a single
mozconfig so they can be set more consistently.
Use this to make unsetting toolchain defines universal in these
tasks, fixing fallout from bug 1283898 which defined CARGO and
RUSTC everywhere, conflicting with --disable-compiler-environment
just like CC and CXX were conflicts in some artifact tasks.
MozReview-Commit-ID: 4SbxByjClQb
--HG--
extra : rebase_source : d8a48fd2192ceb5ece76c827e2243ae784b991cb
Mike Hommey
Chris Manchester
Ralph Giles