Prefer viable nightly Rust builds for CI (#1379)
Prefer viable nightly Rust builds for CI (#1379)

## Proposed Changes

CI is failing on PRs because of a regression in nightly Rust. This change forces `rustup` to install a version of the nightly compiler that can at least build `rustfmt`, which should reduce the frequency of this happening.

## Additional Info

Example failing run: https://github.com/sigp/lighthouse/runs/896694659

Alternatively, we could allow CI to succeed even when `cargo-udeps` fails, but I think we should avoid doing that unless we really have to.




sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse