Add utils function to create REZ_{foo}_ROOT environment variable

[ColinKennedy]

Describe your feature request here.

Motivation Probably the tiniest feature request for Rez ever but I need to convert a Rez package family name from "foo.thing" to an environment variable. I see there's

https://github.com/nerdvegas/rez/blob/1ecc85f638d11d70ed78d4bd9c5cdc6f32ac58c4/src/rez/resolved_context.py#L2040

I'd like that as a standalone function, probably just under rez.utils, so I don't have to hard code it in my own code.