fix: improve error message if build backend crashes

[baszalmstra]

This PR improves the error message when a build backend crashes during the getMetadata call. Instead of showing a cryptic error the output of the build backend is displayed:

main:

Error:   × failed to extract metadata for package 'root'
  ╰─▶   × pipe of stderr stopped earlier than expected

This PR:

Error:   × failed to extract metadata for package 'root'
  ╰─▶   × the build backend (pixi-build-python) exited prematurely.
        │ Build backend output:
        │
        │
        │  ╭─ Resolving environments
        │  │
         ...
        │ thread 'main' panicked at F:\projects\rattler-build\src\metadata.rs:466:14:
        │ Build string is not resolved
        │ note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
        │  │
        │  ╰─────────────────── (took 0 seconds)

[Hofer-Julian]

Looks good to me 💯