Switch to Preferences.jl, remove Pkg.build step

[simonbyrne]

We should be able to now replace Pkg.build and our custom environment variable stuff with the new Preferences.jl. This should also make it easier to provide cluster-specific MPI implementations.

Although discussed in other issues, I don't think we have a concrete issue for this.

1. Figure out some sort of preferences subpackage (Add a lightweight MPIABI sub-package that controls the MPI ABI #529)
2. Update Yggdrasil packages to use preferences system (@vchuravy has a draft PR MPI: Experimental platform tag JuliaPackaging/Yggdrasil#4073)
3. Move any remaining code generation steps to Scratch.jl

[vchuravy]

Now on master