And require it for taskcluster build already, because it doesn't harm and lets
me put all the yml changes in the same commit.
I gave up cross-compiling for OSX after a few tries and after realizing it
wasn't enough with cctools and such, but that I also needed the Mac SDK, for
which I don't have permission...
Differential Revision: https://phabricator.services.mozilla.com/D2664