add wasm32-unknown-emscripten to the conda-forge build of rust

[DerThorsten]

The rust build from conda-forge is missing the wasm32-unknown-emscripten-target. Therefore we cannot use the package as it is for emscripten-forge and need to maintain our own recipe.

Potential difficulties /problem:

• (some?) python packages with rust extensions required rust-nighty to be used over non nightly builds, but that
  might not be relevant.
• Are all rust compilers working with any emscripten version we are using?

[anutosh491]

I guess this would suiffce ?
conda-forge/rust-feedstock#213

Can get rid of the R recipe if that's the next step here

[anutosh491]

@DerThorsten curious as to what work is left here !

[DerThorsten]

I guess this would suiffce ? conda-forge/rust-feedstock#213

Can get rid of the R recipe if that's the next step here

Wait ? R or Rust?

[anutosh491]

Ahhh sorry my bad. I meant rust there

[DerThorsten]

@anutosh491 best would be to just try it out. If the conda-forge recipe is able to compile the rust packages here, we can get rid of our own recipe