[DX Streamline] Do not hardcode the versions of tools in the CI

[galargh]

#464 (comment)

E.g. the version of rust used in CI should be sourced from configuration files rather than hardcoded in the workflow definition.

[vmx]

To make things simpler, I propose reverting the rust-toolchain.toml file back to just a rust-toolchain file which contains the version only. This way CI can just cat file in order to get the version instead of having the need of parsing the TOML file.