windows-ci: temporarily allow Windows jobs to fail
We have unfortunately ran out of free credit on the runners we were using from
OVH for the Windows CI. We will be disabling the two remaining ones on the 30th
of September, hence we need the CI to pass even if Windows jobs cannot start as
a temporary measure. Hopefully we can find another way of getting Windows
runners soon.
Differential Revision: https://phab.mercurial-scm.org/D11499