Builds run `cargo metadata` for all platforms

[WhyNotHugo]

Builds run cargo metadata --manifest-path Cargo.toml without a --target argument. This requires fetching metadata for all platforms.

Downstream (at least on Alpine) builds use the pattern:

# In the source-fetching stage
cargo fetch --locked

# In the build phase
export CARGO_NET_OFFLINE=true
gpep517 build-wheel --wheel-dir .dist …

The above separates fetching sources and compiling the package with those pre-fetched sources.

This fails for packages with py3-setuptools-rust, because it executes cargo metadata which, by default, wants metadata for packages for all targets.

I believe only the current target should be fetched instead (e.g.: rustc --print host-tuple).