Allow building binaries in metadata-only mode

The approach described in https://github.com/bazelbuild/rules_rust/issues/1649 is a great way to get an equivalent of fast `cargo check` on `rust_library` targets. However, on `rust_binary` targets it runs codegen too, not just generating metadata.

This seems to be because the `rust_library` target specifies a metadata output (https://github.com/bazelbuild/rules_rust/blob/d9bbb1d57bef05b3a38a08310bd3760a0e413835/rust/private/rust.bzl#L198) but the `rust_binary` target does not (https://github.com/bazelbuild/rules_rust/blob/d9bbb1d57bef05b3a38a08310bd3760a0e413835/rust/private/rust.bzl#L252).

Would it be possible to produce a metadata output from `rust_binary` too?