Cache Python downloads by default in `python install` tests (#14326)
Adds a cache bucket for Python installs and uses it by default during
tests, extending the opt-in cache added in
https://github.com/astral-sh/uv/pull/12175
Updates the `python_install` tests to use a shared cache for Python
installs. This reduces the `python_install` test runtime on my machine
from 23s -> 17s. The difference should be much larger on machines with
slower internet and less cores for test workers :) This should also
improve stability in CI by reducing reliance on the network during test
runs, see #14327
