chore: pin rustc version with rust-toolchain

[winstxnhdw]

We can pin the Rust toolchain version with a rust-toolchain.toml to avoid situations like #5981

[winstxnhdw]

CI should fail until #5981 is merged and this PR is rebased.

[winstxnhdw]

It seems like it is conflicting with the dtolnay@rust-toolchain action. Maybe we should just pin the action instead?

[davidhewitt]

I would prefer to not have the toolchain file, see Rust-for-CPython/cpython#4 (comment) which matches my sentiment about having the project pull a toolchain locally be annoying

Pinning CI with an automated upgrade PR mechanism seems like my preferred approach

[winstxnhdw]

Okay, I gave a few attempts but I think this will need quite an extensive change. My plan was to interpolate all the Rust toolchain versions in ci.yml using the repository variables. Then with another scheduled workflow, we can check for the latest toolchain releases and compare it with the current toolchain version. Afterwards, we turn ci. yml into a workflow_call that will accept the toolchain version as an input. This is needed so that we can test whether the new toolchain version successfully builds. If all is well, we can use the gh cli to update the toolchain version and submit a PR.

It's quite involved so I think I will close this PR for now and work on this later or after some more discussion.