Does not respect +toolchain argument to cargo.

Running `cargo +nightly build` builds with rustup's "nightly" toolchain.

However, xargo seems to ignore this option; when it's passed, xargo doesn't even attempt to build libcore:

```
# rustup default stable
# xargo +nightly build --target simple
<...>
error[E0463]: can't find crate for `core`
  |
  = note: the `simple` target may not be installed
<...>
```