add(build): Add a rust-toolchain.toml file

[arya2]

Motivation

Cargo should automatically use the right version of rustc when a rust-toolchain.toml file is present.

Specifications & References

https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file

Solution

Adds a rust-toolchain.toml that sets the channel to the latest stable release.

Tests

This was used for convenience in an early version of zebra-launcher that relied on the nightly release for artifact dependencies and worked as expected.

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.