rustup: let RUSTUP_PREFIX env override default prefix.

rustup: let RUSTUP_PREFIX env override default prefix.

The idea here is if you don't want rust in /usr/local
you can put something like this is your .profile:

export RUSTUP_PREFIX=$HOME/.local/rust
export PATH=$PATH:${RUSTUP_PREFIX}/bin
export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:${RUSTUP_PREFIX}/lib

Then when you run rustup, it will update the install
in ${RUSTUP_PREFIX} without having to remember to pass
an explicit --prefix argument every time.