Switch from multirust to rustup

Created by: s22h

I'd like to start a motion to move from multirust to rustup in bootstrap.sh.

According to the multirust repository it is not maintained anymore and recommends to migrate to rustup, which is an official Rust project.

The change shouldn't be too difficult (at least on Linux and OSX, I don't know how it's handled in Windows), since the syntax is the same as in multirust.