Runnable env per build configuration (debug, release)

Hello!

First I would like to say a big thank you for this wonderful extension, really makes working with Rust in VSCode a breeze once a bit of configuration is done 🙂

I'm currently facing a case where we use different RUSTFLAGS between our debug and release builds. rust-analyzer seems to be using mostly the debug build, I tried setting some RUSTFLAGS (the ones for the release profile) through the runnableEnv config, this invalidated the build cache as there was a mismatch between the standard debug flags I use to build stuff in my terminal and the ones (for release) that rust-analyzer used as I set them in the configuration.

So the question is: how hard would it be to have runnableEnvs on a per config basis (debug, release) and more (?) (test, bench) not sure if it makes sense to have additional envs for test and benches, but I guess it could be useful.

Maybe it's already doable and I don't see it!

Otherwise maybe you can point to the relevant code in here and I might be able to contribute something if it's not a high priority feature!

Cheers

Start by tracing rust-analyzer's runnableEnv configuration and how runnable commands select debug and release build configurations. Compare the existing environment handling with the requested debug, release, test, and bench cases; done means each supported configuration can use its own environment without invalidating unrelated build caches.