allow nested config env override

WP [#17225](https://community.openproject.org/work_packages/17225)
Nope. Each iteration goes deeper into the hash and builds the structure step by step.
I.e.

```
value = 'Berliner Str.'

# 1
path' = [:address, :street]
key = :user
new_config = {user: {}}

#2
path' = [:street]
key = :address
new_config = {user: {address: {}}}

#3
path' = []
key = :street
new_config = {user: {address: {street: 'Berliner Str.'}