Keep build backend output off metadata stdout

[marko1olo]

Fixes #2069.

When Hatch reads metadata through a non-Hatchling build backend, the PEP 517 hook runs in a child process. Some backends, notably setuptools/setuptools-scm, print build progress such as running egg_info to stdout while preparing metadata. That output was forwarded directly to Hatch's stdout before Hatch printed the requested version/metadata, which makes commands like hatch version hard to use from scripts.

This redirects backend stdout to stderr for the standard frontend runner. Hatch still preserves backend diagnostics, but stdout remains reserved for Hatch's actual command result.

I verified this with a setuptools-scm sample project: after this change, hatch version stdout is only 1.2.3, while running egg_info remains visible on stderr.

Tests run:

• python -m ruff check src/hatch/project/frontend/scripts/standard.py tests/project/test_frontend.py
• python -m ruff format --check src/hatch/project/frontend/scripts/standard.py tests/project/test_frontend.py
• python -m pytest tests/project/test_frontend.py tests/cli/version/test_version.py::test_other_backend_show tests/cli/project/test_metadata.py::test_other_backend -q
• python scripts/validate_history.py
• git diff --check

[marko1olo]

Local verification on b081f534:

• PYTHONPATH=C:\hades\oss\hatch\src;C:\hades\oss\hatch\backend\src C:\hades\oss\.venvs\hatch\Scripts\python.exe -m pytest tests\project\test_frontend.py::TestPrepareMetadata -q -> 5 passed
• git diff --check origin/master..HEAD passed

Note: I had to set PYTHONPATH explicitly because this shared local venv was still pointing at another Hatch checkout from a different branch.