Make a `rust-toolchain.toml`

[ControlCplusControlV]

As seen in in the Cargo Book fuel-core and all associated repos should have a rust-toolchain.toml specifying a toolchain which the repo is tested and built with

[Voxelot]

It seems like this is more heavily recommended when you need to pin nightly versions of rust.

[ControlCplusControlV]

Yeah, apparently this can hook with github actions, but is currently blocked by actions-rs being unmaintained, which is explained more here . Closing for aforementioned reasons there though