Compile go from source for cross compilation

This takes a little (lot) longer, but is necessary if we want to cross
compile for other platforms (we do).

Initially I had a call to goxc -t which builds the necessary tooling to
cross compile for those other platforms, but that takes like 10 minutes
and it gets handled automagically the first time you cross compile
anyway.
Don't use -u in go get, it takes too long