cross-compilation/deployment of subtasks

[Gankra]

Supporting cross-compilation of things like the main app (Step 3) is desirable for a few reasons:

• Finding issues in cross-compilation toolchains themselves. Mozilla is building for every platform under the sun from x64 linux, so if there's issues unique to that, shaking it out would be good.

• Some platforms are difficult to run our toolchains on, like "all of our compilers". Like iOS is probably impossible, and Android is a pain in the ass. Cross-compilation is simply The Norm for those platforms.

I think only Step 3 really needs/wants this, but being able to mix-and-match would be fun/cute.

#9 would probably go a long way towards this since mozilla already has workflows like this. That said I don't know how strongly we want to couple with their workflows.

[Gankra]

An alternative to #9 would be spinning up VMs of some kind, but I don't know where that falls on the practical/hellish/useless scale.