chore: re-enable nilnil lint for models/actions/task.go (#12768)

I added sentinel error values for jobs not being found and no jobs being updated by the `CreateTaskForRunner` function. This avoids the nilnil antipattern and allows the nilnil lint to be enabled for the `models/actions/task.go` file again.

The handling of these new errors was added to the `PickTask` function in `services/actions/task.go`.

Related issue: https://codeberg.org/forgejo/forgejo/issues/11261

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12768
Reviewed-by: limiting-factor <limiting-factor@noreply.codeberg.org>
Reviewed-by: elle <0xllx0@noreply.codeberg.org>

.golangci.yml

@@ -189,9 +189,6 @@ linters:
         text: "(ST1005|ST1003|QF1001):"
 
       # TODO: eventually remove this section entirely
-      - path: models/actions/task.go
-        linters:
-          - nilnil
       - path: models/activities/action_list.go
         linters:
           - nilnil