Replace multirust with rustup.sh

[RoxasShadow]

This PR aims to solve #21. Both rustup.sh and cargo will be installed in cached folders in case they are not installed yet. At every deploy we run rustup update to update rustup.sh itself and Rust to the latest versions, whether they're present.

Unfortunately I had problems running most of the rustup.sh commands without explicitly passing our ENV variables to them (and this is why I had to wrap every rustup.sh command inside cmd_exec()) – local variables and exported variables didn't work for me :(