Supporting adding to PATH on non-Windows platforms

[LPGhatguy]

We can follow what Cargo does here.

They append to ~/.zshenv and/or ~/.profile the following:

. "$HOME/.cargo/env"

and then include this script, ~/.cargo/env:

#!/bin/sh
# rustup shell setup
# affix colons on either side of $PATH to simplify matching
case ":${PATH}:" in
    *:"$HOME/.cargo/bin":*)
        ;;
    *)
        # Prepending path in case a system-installed rustc needs to be overridden
        export PATH="$HOME/.cargo/bin:$PATH"
        ;;
esac

This seems reasonable to me.

[LPGhatguy]

Solved by #16.