use remotes.d path/files to configure default flatpak remotes

@jane400 pointed out in !4756 (closed) that we can use remotes.d to configure flatpak remotes instead of adding them manually in a post-install, this is a MUCH better mechanism that I hadn't know about until her comment :)

https://docs.flatpak.org/en/latest/flatpak-command-reference.html#flatpak-remote

System-wide remotes can be statically preconfigured by dropping flatpakref files into /etc/flatpak/remotes.d/.