Include the parent interpreter in Python discovery when `--system` is used (#7440)
Closes https://github.com/astral-sh/uv/issues/7417
Tested with this epic blurb
```
❯ uv pip install --python /Users/zb/.local/share/uv/python/cpython-3.13.0rc2-macos-aarch64-none/bin/python3 . --break-system-packages --reinstall
Resolved 1 package in 0.91ms
Built uv @ file:///Users/zb/workspace/uv
Prepared 1 package in 2m 48s
Uninstalled 1 package in 1ms
Installed 1 package in 1ms
- uv==0.4.10
+ uv==0.4.10 (from file:///Users/zb/workspace/uv)
❯ /Users/zb/.local/share/uv/python/cpython-3.13.0rc2-macos-aarch64-none/bin/python3 -m uv pip install --system -v httpx
DEBUG uv 0.4.10
DEBUG Searching for Python interpreter in system path
DEBUG Found `cpython-3.13.0rc2-macos-aarch64-none` at `/Users/zb/.local/share/uv/python/cpython-3.13.0rc2-macos-aarch64-none/bin/python3` (parent interpreter)
DEBUG Using Python 3.13.0rc2 environment at /Users/zb/.local/share/uv/python/cpython-3.13.0rc2-macos-aarch64-none/bin/python3
```