lang/rust: Let the framework extract the bootstraps

It is possible for lang/python37 to be built in such a way that it
installs an unloadable lzma module which then causes Rust to fallback
to trying to fetch/extract the tar.gz bootstraps instead.

As a workaround and since it also simplifies some things, let the
ports framework extract the bootstraps and "install" them under
WRKDIR. We point the build to them in config.toml. This is similar
to how things are hooked up in lang/rust-bootstrap and Rust will
then not try to fetch and extract the bootstraps on its own.

PR: 243766
Reviewed by: mikael
Differential Revision: https://reviews.freebsd.org/D24582