Please support a "system" toolchain

[joshtriplett]

I'd like to be able to test things using my system-installed version of Rust, and also use nightly Rust via rustup. To support this, I'd love to have support in rustup for a "system" toolchain, which would allow cargo +system, or setting system as the default and using cargo +nightly.

When using the "system" toolchain, the various rustup-managed binaries would simply forward to the corresponding system binaries.

[joshtriplett]

Note that rustup toolchain link system /usr provides roughly the desired behavior; I'm simply suggesting that rustup support doing that by default for a system installation of Rust so that it works out of the box.