allow nested config env override

WP [#17225](https://community.openproject.org/work_packages/17225)
avdi would tell you to inject the env hash

```ruby
def load_overrides_from_environment_variables(..., env_hash = ENV)
  env_hash['BLA']
end
```

then, in your tests you would say

```
let(:env_hash) { { 'BLA' => '1' } }
load_overrides_from_environment_variables(..., env_hash)
```