Prefer system Python installations over managed ones when --system is used

[zanieb]

This fixes a regression from 0.8.0 from #7934 and follows #15059

The regression is from this change, which was made because we'd otherwise (with the other changes in that pull request) filter out managed Python interpreters found in virtual environments.

When --system is used we'll convert the default Python preference of managed to system which avoids things like uv pip install --system targeting a managed Python installation.

The basic test is

uv python install
uv pip install --system anyio

Prior to this change, we'd read a managed interpreter from our managed installation directory and target that. After this change, without #15059, we'd read a managed interpreter from the PATH and target that. Both of those experiences are bad, because the managed interpreters are marked as externally managed. After this change, with #15059, we properly target the system interpreter.

Since we use system instead of only-system, if there is not a system interpreter we'll still retain our existing behavior and use a managed interpreter. This should limit breakage from the change. Given the source of the regression, we could probably use only-system here. I don't feel strongly. I think the main benefit of doing so would be that we'd omit the check for managed installations in error messages when an interpreter cannot be found?

We can't really add test coverage here because the test suite always has externally managed interpreters :)