Some specs fail in rails5 branch with errors like

```
1) Group#secret_variables_for when the ref is not protected contains only the secret variables
    Failure/Error: variables = Ci::GroupVariable.where(group: list_of_ids)

    NoMethodError:
      undefined method `arel_table' for Ci::Group:Class
```

This commit fixes it.

Goldiloader is great, but has several issues with has_many :through relations:

* https://github.com/salsify/goldiloader/issues/12
* https://github.com/salsify/goldiloader/issues/14
* https://github.com/salsify/goldiloader/issues/18

Rather than try to figure out which applies in each case, we should just do the
drudge work of manually disabling autoloading for all relations of this type. We
can always use regular preloading for specific cases, but this way we avoid
generating invalid queries through Goldiloader's magic.

The purpose of this memoization is to make getting persisted environment
name, and related scoped variables, a little more performant task,
because it can be invoked multiple times.

This makes it possible to access static build variables even when an
object is not persisted yet. This will allow us to evaluate build
variables using only/except policies before deciding whether we actually
want to persist the build in the database, or not.