Some of the CI tests use WSL. This switches the WSL distribution
from Debian to Alpine, which might be slightly faster. For the way
it is being used here, the main expected speed improvement would be
to how long the image would take to download, as Alpine is smaller.

(The reason for this is thus unrelated to the reason for the Alpine
docker CI test job added in gitpython-developers#1826. There, the goal was to test on a
wider variety of systems and environments, and that runs the whole
test suite in Alpine. This just changes the WSL distro, used by a
few tests on Windows, from Debian to Alpine.)

Two things have changed that, taken together, have unblocked this:

- Vampire/setup-wsl#50 was fixed, so the
  action we are using is able to install Alpine Linux. See:
  gitpython-developers#1917 (review)

- gitpython-developers#1893 was fixed in gitpython-developers#1888. So if switching the WSL distro from
  Debian to Alpine breaks any tests, including by making them fail
  in an unexpected way that raises the wrong exception, we are
  likely to find out.