[💡 FEATURE REQUEST]: Allow defaults in env var expansion of server.env config

[devnev]

Plugin

Server

I have an idea!

The current handling of server.env configuration does a plain os.Expand(_, os.Getenv()). Unlike other parts of the roadrunner config, this doesn't allow for defaults in the form ${ENV:-default}.

It would be great to allow defaults, or even go one step further and use https://github.com/drone/envsubst both here and in the general configuration for some more advanced substitution options.