Hi Guix Developers, I want to add a page to the installer to allow selection of substitute mirrors. In particular, from my ISP, the SJTU mirror is significantly faster (1MB/s->5MB/s) than directly from `ci.guix.gnu.org` (4kB/s->40kB/s), so I think putting that option to the installer would be beneficial to others in similar circumstances.
However, the current `(gnu installer services)` assumes that snippets are added to a list of services. To implement substitute mirrors, I need to insert a `modify-services` form with a `(guix-service-type config => (guix-configuration (inherit config) (substitute-urls <whatever>))`. What I want to propose is something like this: * Add a `snippet-type` field to `<system-service>`, default `'service-list`. * The field can instead be set to `'modify-services` to mean that the snippet is to be in a `modify-services` form. * In `system-services->configuration`, filter services according to `snippet-type` and put the snippets in the appropriate parts of the `service` form. * Add entries for the SJTU mirror (with fallback to the original `ci.guix.gnu.org`) and the default unmirrored `ci.guix.gnu.org`, with `system-service-type` of `'substitute-urls`. * Then in `(gnu installer newt services)`, add a page for `'substitute-urls`. What do you think? Thanks raid5atemyhomework
