Configurable installation directory for binaries

[mzabaluev]

As described in rust-lang/docker-rust#17, $CARGO_HOME/bin may be a poor choice of installation directory for binaries inside a container, because users of the container image may want to bind-mount the entire $CARGO_HOME to a host directory to preserve Cargo caches and authentication secrets between container runs. The installation location should be configurable, either with an environment variable or via $RUSTUP_HOME/settings.toml.

[sbrl]

I second this. I'd like to install rustup, but I don't currently feel I can because I can't suitably control where rustup puts things.

[mzabaluev]

One complication is that rustup also manages $CARGO_HOME: it tries to delete it on uninstall.
I'm not sure what the behavior should be when the binary install directory has been configured to be somewhere else.

[Diggsey]

There's already a long-term issue for configurable directories (#247)

The specific motivation in this ticket is not particularly compelling: cargo is the tool which defines $CARGO_HOME/bin as the place to put binaries. If users don't want to cache binaries they should not bind that path to the host.