chore: pin Rust 1.60.0

[ethowitz]

Description

Rust 1.61.0 uses substantially more memory than 1.60.0 in our CI builds, preventing them from completing (and preventing us from merging anything new). This PR pins Rust 1.60.0 until the issue is resolved upstream.

Upstream issue: rust-lang/rust#97549

[ethowitz]

Using cimg here upgrades us to using CircleCI's new build images (circleci uses the legacy build images). More info here: https://discuss.circleci.com/t/legacy-convenience-image-deprecation/41034

[jrconlin]

Heh, I was experimenting to see if there was a way we could convert the rust version to a Variable, because we pin to it for a bunch of different stuff (circleci, Dockerfile, etc.) We might be able to use an environment variable for it, but everything seemed complicated and prone to failure.

[taddes]

@jrconlin Where in such a large collection of crates or in such a project project would env vars typically sit? At the root, or say in the build directory? Is it a concern that said variable could be missed and result in regressions that are hard to find or fix? It does sound like a good idea though

[jrconlin]

Yeah, that's the biggest problem. Technically, the circleci config file should be the one that's at the top level since it fires off all the other containers, but it's not super easy to deal with. It might be possible to store them as environment variables, but then you'd have to go into the circleci admin panel to update the values, but that's a pain and prone to forgetting. There might be other methods as well, but they're equally complex and fragile.