Consider presence on the `PATH` an opt-in to Python pre-releases

[zanieb]

From @henryiii in #7300 (comment)

Yes, the issue wasn't VIRTUAL_ENV, but rather that we were making the venv assuming it would use the Python on the top of the PATH. cibuildwheel 2.21.1 fixes this.

It's really weird, though, that it no longer takes the Python at the top of the PATH when --system is used. If someone goes to the trouble to put Python in the PATH, intentionally skipping over it is really surprising; and if you type python, you don't get the one you just installed to. For example:

- uses: actions/setup-python@v5
  with:
    python-version: ${{ matrix.python-version }}
    allow-prereleases: true
- uses: astral-sh/setup-uv@v3
- run: uv pip install --system some_lib
- run: python -c "import some_lib"

now fails only on 3.13 because uv installed to the 3.12 instead of the 3.13 that's been prepared by setup-python. The python above is 3.13, while uv is skipping it and installing into 3.12. (I've seen this failure somewhere but have forgotten which repos it affected).

I understand this makes sense with managed Python (where you have a bunch of Pythons and uv's just picking out the best one), but it is very surprising to have it do this to the Python on PATH.

We may want to consider allowing --system to opt-in here, but I'm a little hesitant to have different discovery behaviors.

cc @carljm / @AlexWaygood who suggested to me that we should not select Python pre-releases by default.

[henryiii]

I don't like handling pre-releases differently from full releases in general - if someone is helping test pre-releases, making it harder for them isn't very helpful. We want to encourage pre-release to be easy to test and use like regular releases (especially RC's). The only exception I know of is the original motivation: if you have installed several managed Pythons, defaulting to the latest non-prerelease is a good default. But these aren't on the PATH. What about only having special handling for commands that pick up a managed release automatically, and letting PATH work normally? Is that possible?

- uses: actions/setup-python@v5
  with:
    python-version: "3.13"
    allow-prereleases: true
- run: python --version  # 3.13
- run: uv venv # I would kind of expect Python 3.13 here

- run: uv python install 3.11
- run: uv python install 3.12
- run: uv python install 3.13
- run: python --version # This is whatever's already present, or nothing
- run: uv venv  # I would not expect Python 3.13 here, either system or 3.12

[zanieb]

It's definitely possible — we just need to align on the behavior.

I believe #7470 would achieve the behavior here.