Read version from Cargo.toml

[nichmor]

Right now for the pixi-build-rust backend we duplicate the version both in pixi.toml [package] and in the Cargo.toml.

I think it would be nice to read the version directly from the Cargo.toml ( alongside other fields like description etc..)

We only do this in the backend if no version is provided over the protocol. We want to be explicit about this behavior but delegated this to PFX-684.