Add Python distribution mirror environment variables

[Lucas-FManager]

Problem

Hatch can already override individual managed Python distributions with HATCH_PYTHON_CUSTOM_SOURCE_<NAME>, but enterprise mirror setups need a single base URL override for the built-in CPython/PyPy sources. Today that means maintaining one env var per distribution, and the built-in URLs stay coupled to the upstream hosts.

Approach

• Add HATCH_PYTHON_INSTALL_MIRROR for CPython standalone downloads and HATCH_PYPY_INSTALL_MIRROR for PyPy downloads.
• Rewrite only known built-in source base URLs and preserve the archive path suffix.
• Keep explicit source arguments unchanged.
• Keep per-distribution custom sources higher priority than the mirror behavior.
• Parse PyPy versions from the archive name so mirrored URLs do not need to contain /pypy/.
• Document the new mirror variables and correct the custom source env var name in the virtual environment docs.

Security

The new variables are user-controlled in the same trust boundary as the existing custom source variables. Rewriting is intentionally limited to known built-in upstream bases, and users are told to point mirrors only at trusted sources.

Verification

• .\.venv\Scripts\python.exe -m pytest tests/python/test_resolve.py -q -> 13 passed, 10 skipped
• uvx ruff format --check src/hatch/config/constants.py src/hatch/python/resolve.py tests/python/test_resolve.py -> passed
• uvx ruff check src/hatch/config/constants.py src/hatch/python/resolve.py tests/python/test_resolve.py -> passed
• git diff --check -> passed

Fixes #2236