Allow building binaries in metadata-only mode

[reinerp]

The approach described in #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 (

rules_rust/rust/private/rust.bzl

Line 198 in d9bbb1d

metadata = rust_metadata,

) but the rust_binary target does not (

rules_rust/rust/private/rust.bzl

Line 252 in d9bbb1d

edition = get_edition(ctx.attr, toolchain, ctx.label),

).

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