ci: migrate away from actions-rs

[toidiu]

Mirroring the issues from s2n-quic

Problem:

The entire organization has been archived: https://github.com/actions-rs

Solution:

Most of our usage is just setting up the correct toolchain. Funny enough, rustup is already included in the GHA build image. So my vote would be to just invoke the rustup commands directly to avoid any dependencies.