install-path array

[Gankra]

You should be able to pass a series of strategies to install path, like:

install-path = ["$CARGO_HOME", "~/.cargo"]

This precise example would have equivalent semantics to the current CARGO_HOME builtin.

[Gankra]

Updated text to assume #934

[Gankra]

(NB the CARGO_HOME builtin may ultimately want a mild divergence to avoid copying anything but binaries in, but hopefully you get what i mean)

[Gankra]

solution: the CARGO_HOME builtin is really weird to want on a cascade like this, so error out if it's in a list of length > 1 (users can ask for it as a feature if this is a problem)

[Gankra]

Implemented in #947