Use case: downstream users may want to override the GNU mirror URL,
as the main ftp.gnu.org repository has caused slow downloads. Now it is possible to
perform e.g.
```nix
((import ./default.nix).extend {
modules = [{ config.aux.mirrors.gnu = "https://www.nic.funet.fi/pub/gnu/ftp.gnu.org/pub/gnu/"; }];
}).config.exports.resolved.packages
```
Reviewed-on: #14
Reviewed-by: Jake Hamilton <jake.hamilton@hey.com>
Co-authored-by: Aleksi Hannula <ahannula4+auxolotl@gmail.com>
Co-committed-by: Aleksi Hannula <ahannula4+auxolotl@gmail.com>
