Cargo commands are invoked through rustup for uninstalled toolchain (WSL)

[KalitaAlexey]

As per #289, #315 introduces a new configuration parameter, "rust.shell.kind.windows" which can be used to specify what kind of terminal is used.
If the user uses WSL and Cargo commands are executed through rustup, then "rust.rustup.toolchain" will be used as a toolchain which rustup will be invoked with.
The problem is that the extension allows to choose only toolchains installed on Windows (not WSL) and hence rustup will be invoked with some not installed toolchain.
The workaround is to install the toolchain on Windows which will be used in WSL.
An example: nightly-x86_64-unknown-linux-gnu
The motivation is described in #315