fix(cmake): better --fresh support

[henryiii]

Description

This restructures the venv creation support. You can now run multiple times safely with different interpreters by adding --fresh. Before:

cmake --preset venv -DPYBIND11_CREATE_WITH_UV:PATH=~/.pyenv/versions/3.14.0b1t/bin/python
cmake --build --preset venv --target cpptest

rm -rf build .venv  # <-- Bug to forget this!
cmake --preset venv -DPYBIND11_CREATE_WITH_UV=3.13t
cmake --build --preset venv --target cpptest

After:

cmake --preset venv -DPYBIND11_CREATE_WITH_UV:PATH=~/.pyenv/versions/3.14.0b1t/bin/python
cmake --build --preset venv --target cpptest

cmake --preset venv -DPYBIND11_CREATE_WITH_UV=3.13t --fresh
cmake --build --preset venv --target cpptest

It also doesn't interfere with a developer .venv.

Maybe this should be in a distinct folder now, to keep default and venv from interfering?

Suggested changelog entry:

• Restucture venv support to support --fresh, make in build folder