Fix EnvPrefix not being used with aconfigdotenv

[aponad]

Using aconfigdotenv to load env variables from file with EnvPrefix with not work as if it would be with passed ones.

Example test (I will create a separate PR after if this one will be merged):
https://github.com/cristalhq/aconfig/compare/main...aponad:aconfig:add-env-prefix-test?expand=1

[cristaloleg]

Hey, thanks for the PR. Why not add test in here?

[aponad]

Hi,
Tests are in aconfigdotenv, but fix is in aconfig.
We will have to bump aconfig version in aconfigdotenv for them to pass.