feat(flag): resolve env's from config file

[DmitriyLewen]

Description

viper doesn't resolve env's from config file - https://github.com/spf13/viper/issues/315. But we wrap some logic - so we can do it.

Most likely, env will be used for string or []string types. So I propose to allow env only for these types now.

We may add logic for other types later (upon user requests)

Discussed in https://github.com/aquasecurity/trivy/discussions/8426

[knqyf263]

Since this is not a bug fix, I think we should discuss if we really need it first.

[knqyf263]

Closing as I'd like to gather more feedback from the community first in https://github.com/aquasecurity/trivy/discussions/8426.