We need the Rust snapshot (stage0) for the build environment not the
host environment. This is not affecting us currently because we are only
building native versions of this today but should improve the behavior
of #81 or if any Rust components are built for the target machine.
The host system where things are being built really should control what
toolchain is used for stage0. This in theory should improve #23 but I'm
not sure how to specify a hash per host sys.
This drops the ability to supply your own rust through the PACKAGECONFIG
local-rust option. This isn't tested and we really need to better expand
support for build arches. At the same time this simplifies how the rust
stage0 snapshot is extracted and used by the build system.
Doug Goldstein