CI uses nightly version which disallowed the toolchain file to use its own version

[Daksh14]

either change CI nightly version or use same versions in toolchain file and CI

[ZER0]

The CI should automatically pick the rust-toolchain file if present, as we do in other projects (see Rusk for example), we shouldn't have anything hard-coded; so that the only "source of truth" is the rust-toolchain file. You can also thinking of change it in its toml version, that gives us more flexibility (when introduced the first time the toolchain file, the toml wasn't supported yet, but now it's been a while): https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file