Add cross-compilation environment variables (mkCrossToolchainEnv) to devshell by default

[axelkar]

Is your feature request related to a problem? Please describe.
I'm trying to build a Rust project manually with cargo b, but it requires the MinGW pthreads fix, which is provided by mkCrossToolchainEnv.

Describe the solution you'd like
Add them by default, like in mkCargoDerivation.

Describe alternatives you've considered
Manually use mkCrossToolchainEnv and take the wanted environment variables? I can't figure out a way to get the configureCargoCommonVars hook to run in devshell

[DavSanchez]

I'd be interested in having this, to somehow streamline doing cargo run or cargo test for Windows targets via wine automatically from the devShell, or something like that. Haven't put much thought into that but I think it might be possible.