Automatically create production update MRs

See merge request !318

bump nixpkgs version

See merge request !320

Re-add stages

See merge request !317

So expand it ourselves

GitLab docs explicitly say `needs` does not require stages but GitLab actual behavior is:

```
Found errors in your .gitlab-ci.yml:

    update-staging job needs system-tests job, but system-tests is not in any previous stage
    update-staging job needs morph-build-staging job, but morph-build-staging is not in any previous stage

```

Automate updating nixpkgs pin in develop branch once a week

See merge request !305

The default is `when: "on_success"` which is equivalent to "always" since we
have no stages.

Some terminal code stripping logic added though