Undocumented removal of core.longpaths setting in Windows helper images

gitlab-runner 17.8 helper images no longer sets core.longpaths true, resulting in runner failures on git checkout.

It's not really an issue if this was intentional (I've just updated the pre_get_sources_script), but not documenting it isn't particularly fun.

It looks like this was done in this commit: f37c7b6d (at least, that's where the config in this repo disappeared in favour of that in https://gitlab.com/gitlab-org/ci-cd/runner-tools/base-images (?)