Tweak recurrent visit scheduling policies (& related changes)

• origins generated by the nixguix lister don't have a last_update date, no need to try to schedule visits with any other policy than origins_without_last_update.

• git origin repartition has been upended by a regression in the GitHub lister (swh/devel/swh-lister#4710 (closed)). For now, tweak the weights to be able to schedule visits

This also contains the changes done in production to work around the webhooks not being able to keep up with the new rate of visits.

helm-diff: https://krkr.eu/tmp/2024-06-10-5KGNtd52A0s/scheduler-tweaks.html

Addresses swh/infra/sysadm-environment#5330 (not in a very satisfactory way...)