Auto merge of #1111 - MarkusPettersson98:rustup-1.21-yaml, r=RalfJung

Updated CI config to reflect deprecation of rustup uninstall

In the same spirit as #1110 😄

This PR from the rustup repository brought me here: https://github.com/rust-lang/rustup/issues/2148

TL;DR With rustup 1.21.0 `rustup install` and `rustup uninstall` are being deprecated in favor of `rustup toolchain install` and `rustup toolchain uninstall`. There's plenty of code/documentation out there that needs to be updated to reflect this.

Thought It would be cool to help, however small the change may be. :)

Keep up the great work!