Automate updating nixpkgs pin in develop branch once a week

This is somewhat related to https://whetstone.private.storage/privatestorage/privatestorageops/-/issues/422 but it doesn't do a develop -> production MR. Instead, it does a feature-branch -> develop MR that updates nixpkgs.

Supposing this works (as it seems to, on staging) most of the same tooling (maybe using GitLab include templates with cleverly crafted variables holes) can be used to make the "develop -> production" MR as well as some PrivateStorageOps MRs for updating the differently-managed (!) nixpkgs pin there (and a similar "develop -> production" automatic workflow).

This goes along with the lovingly hand-crafted "Pipeline Schedules" configuration visible on https://whetstone.private.storage/privatestorage/PrivateStorageio/-/pipeline_schedules by the description of "regular nixpkgs update". That schedule currently points at this branch but once this is merged it needs to be updated to point at develop instead.