Use base executable to set virtualenv Python path (#8481)

See extensive discussion in
https://github.com/astral-sh/uv/pull/8433#issuecomment-2430472849.

This PR brings us into alignment with the standard library by using
`sys._base_executable` rather than canonicalizing the executable path.

The benefits are primarily for Homebrew, where we'll now resolve to
paths like `/opt/homebrew/opt/python@3.12/bin` instead of the
undesirable
`/opt/homebrew/Cellar/python@3.9/3.9.19_1/Frameworks/Python.framework/Versions/3.9/bin`.

Most other users should see no change, though in some cases, nested
virtual environments now have slightly different behavior -- namely,
they _sometimes_ resolve to the virtual environment Python (at least for
Homebrew; not for rtx or uv Pythons though). See
[here](https://docs.google.com/spreadsheets/d/1Vw5ClYEjgrBJJhQiwa3cCenIA1GbcRyudYN9NwQaEcM/edit?gid=0#gid=0)
for a breakdown.

Closes https://github.com/astral-sh/uv/issues/1640.
Closes https://github.com/astral-sh/uv/issues/1795.

crates/uv-python/python/get_interpreter_info.py

@@ -563,6 +563,7 @@ def main() -> None:
         "sys_executable": sys.executable,
         "sys_path": sys.path,
         "stdlib": sysconfig.get_path("stdlib"),
+        "sysconfig_prefix": sysconfig.get_config_var("prefix"),
         "scheme": get_scheme(),
         "virtualenv": get_virtualenv(),
         "platform": os_and_arch,