rust-toolchain & CI usage

[Dinonard]

Pull Request Summary

Added rust-toolchain to make it consistent with Astar repo.

Running rustup show will install the required compiler version as specified in the toolchain toml file. CI updated to use the same approach.

Main benefit is that we have comparable local & CI builds.

Check list

• [ ] contains breaking changes
• [ ] adds new feature
• [x] modifies existing feature (bug fix or improvements)
• [ ] relies on other tasks
• [ ] documentation changes
• [ ] tests and/or benchmarks are included
• [ ] changed API client type definition or chain metadata