Rust libraries contained in gnu/packages/crates-io.scm are not building 
anymore because cargo wants to download crate dependencies inside the 
store.

The attached patch sets the CARGO_HOME environment variable  to "." much 
earlier than it previously was, just after the configure phase. With the 
attached patch all packages in crates-io.scm build without errors.

I hope I did everything right,

Bye,

Giacomo