Support --dns and env for selecting dns backend

[NobodyXu]

So this --dns / ENV feature would be convenient, but I'm all for keeping maintenance simple if there's little demand for it, no need for feature creep 😝 (alternative builds or an option to switch the resolver works just as well)

I think it is relatively easy to implement, and should not be too much of a burden.

That said I'm a bit busy recently and working another relatively large PR related to GH private repo, so it would take some time for me to get around.

Though I will always welcome contributions.

Originally posted by @NobodyXu in #1688 (comment)