Move flake-compat to the NixOS org

[infinisil]

flake-compat is currently a repo under @edolstra's account, even though it's become a core flake utility.

@edolstra Would you consider transferring flake-compat to the NixOS GitHub org, to both solidify its official status, and to let others perform or delegate its maintenance?

Note that because this would newly be an official project, we'd also need the Steering Committee to approve it.