Rust does something fairly different than in 1.7. Instead of just
expecting the tarball to exist, it either expects an already extracted
and ready toolchain, or else it does everything itself.
To work with that, we'll always pass --use-local-rust to ./configure so
that bootstrap.py doesn't try to download anything. We'll either
download and setup a snapshot ourselves, or use the system rust, based
on PACKAGECONFIG[local-rust] as before
Steven Walter
3d0fab8860