buildRustCrate: use less bash for the build script

We can get rid of a bunch of workarounds that were in the build script
before by just passing on the `crateBin` attribute.

Before we converted the list of attributes to a string only to convert
it back in bash during the build phase. We can do the entire looping
through builds in Nix and thus need no conversion and parsing of
attributes over and over again.

The big part that still remains bash is the heuristic that cargo
introduced and that we can't do at eval time.