Respect non-`sysconfig`-based system Pythons (#2193)
## Summary
`pip` uses `sysconfig` for Python 3.10 and later by default; however, it
falls back to `distutils` for earlier Python versions, and distros can
actually tell `pip` to continue falling back to `distutils` via the
`_PIP_USE_SYSCONFIG` variable.
By _always_ using `sysconfig`, we're doing the wrong then when
installing into some system Pythons, e.g., on Debian prior to Python
3.10.
This PR modifies our logic to mirror `pip` exactly, which is what's been
recommended to me as the right thing to do.
Closes https://github.com/astral-sh/uv/issues/2113.
## Test Plan
Most notably, the new Debian tests pass here (which fail on main:
https://github.com/astral-sh/uv/pull/2144).
I also added Pyston as a second stress-test.
