Avoid reinstalling rust on every CI run

[Garandor]

One more optimization could be to use https://github.com/rust-lang/docker-rust-nightly

as in runs_on: rustlang/rust:nightly instead of running rustup at our compute cost on every run

currently as in https://github.com/Manta-Network/Manta/actions/runs/3238290500/jobs/5306447191#step:2:10 we do aws-image-search-pattern: ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*

A quick search if aws has any prebuilt rust images could be helpful

Originally posted by @Garandor in https://github.com/Manta-Network/Manta/issues/743#issuecomment-1207591788