Rust's responsibility to engage in good neighborly behavior

[Nemo157]

@josh

Compatibility needs to take into account that both rustup and cargo have come to share some common directories, which makes it difficult for a version of either to unilaterally move those directories.

Not just rustup and cargo, cargo-deny and cargo-audit have also used CARGO_HOME as their config+cache-dir for some reason. There’s almost certainly other thirdparty subcommands too.