This introduces some env vars to allow running cross-compiled
binaries instead of running things on the builder. Additionally
the `cargo test --features "ipc"` check is modified to be `check`
instead since there are no actual tests being run by that command.
The only thing we lose is a rustdoc example check but we are
checking that on Linux anyway so doing it for Mac is redundant.
Differential Revision: https://phabricator.services.mozilla.com/D19367
--HG--
extra : source : ee403c79877e028c58fa9091dd360fe50a80af37