[gomod] 1.24: Add support for private module fetches

[eskultety]

There's the following bit of information in Go 1.24 release notes:

The new GOAUTH environment variable provides a flexible way to authenticate private module fetches. See go help goauth for more information.

We currently don't know of anyone making use of this functionality, but it's definitely worth exploring once the need for it arises. At least we know Go now provides an easy mechanism for the job which is also compatible with our way of dealing with go - environment variables.