Feature: Expand environment variables in cache_dir config

[hetmankp]

Just like issue #6508 brought up the need for '~' home directory user expansion in the cache_dir option in the configuration file, I think it would be very helpful to also do environment variable expansion.

This will help in creating more generic configuration files that work across multiple projects as well as making it easier to integrate with wrapper tools.

I have previously submitted a PR #7022 for this but have not received any feedback so I thought I might need to create an issue to discuss this first.