Add back rust-toolchain.toml so all dev have a consistent compiling env

[jimmychu0807]

Refer to Substrate example:
https://github.com/substrate-developer-hub/substrate-node-template/blob/main/rust-toolchain.toml

But using this rust-toolchain file causes you to download a new nightly version and recompile your code everyday. So you can fix the compiler nightly version. Example here.

[ytqaljn]

Thank you for your suggestion. We have made the corresponding changes.