Update github actions to use non-deprecated actions

[marsella]

The actions-rs/toolchain action we use to install Rust in our CI is not maintained and uses features that are being deprecated "very soon" by Github Actions. We should try to remove this dependency so we don't get caught off-guard if it stops working.

There's quite a bit of discussion online about alternative options. They include

• dtolnay/rust-toolchain/: a different, maintained action that does the same thing. I think it's best for "caching toolchains that are not installed directly" on the github action server images
• manually updating Rust: I guess the server images used for Github Actions already have Rust installed, so you don't have to go through an action

I'm not sure which of these has better caching behavior, but they both seem reasonable to me.

Similarly, actions-rs/cargo is not maintained but it seems like we shouldn't need to use a whole action for that.

Closing criteria

• Update CI to (a) work and (b) not throw a bunch of warnings

[LordQ1]

Duplicated over to https://app.zenhub.com/workspaces/bolt-team-60886be9e01f13000e361d92/issues/gh/boltlabs-inc/maestro/330 for working; hopefully you will copy the fix.

[marsella]

Copy-able fix in https://github.com/boltlabs-inc/maestro/pull/366

[marsella]

Noting: this should be done by someone internal to Bolt who has access to that repo, and not by a contractor.