chore: be consistent with core crypto

[istankovic]

Use nightly for local dev environment, but stable for CI (except for rustfmt and clippy).

[fewerner]

I have two things:

1. We should probably add a script to update the rust-version of each crate as we did in core-crypto
2. Reason for the nightly toolchain were the formatting flags, I would add these here or in a separate PR and format the repo once.

[istankovic]

I have two things:

1. We should probably add a script to update the rust-version of each crate as we did in core-crypto

I think that is unnecessary since we can just reuse the script from core-crypto (and so avoid having to maintain two scripts).

2. Reason for the nightly toolchain were the formatting flags, I would add these here or in a separate PR and format the repo once.

I did that and it was a no-op -- apparently no formatting changes are necessary.

[fewerner]

Please take a look at the rustfmt.toml of core-crypto. Some flags are missing, we should probably copy that file.

[fewerner]

Thank you!