This avoids an occasional HTTP 403 error updating WSL for WSL2.

Updating WSL2 also takes a bit of time, and this change seems to decrease how long the "Setup WSL" step takes, by about 30 seconds. Both benefits could also be achieved by continuing to use WSL2 but running the Windows jobs on windows-2025 (so long as #2008` is merged). For details on the problem this is fixing and possible approaches to it, see #2008 (review).

Due to #2004, the Cygwin test job should be expected to fail here until #2009 is merged. Rebasing this could then make everything pass, but it is only in the non-Cygwin Windows jobs that we ever use WSL, so such a rebase is probably not necessary.