pre-installed flatpak remote doesn't work

We pre-configure the flathub remote for flatpak when building images, however this seems to be broken at least on the systemd branch (but likely affects edge too).

On a fresh install, running flatpak remotes shows flathub as expected, however flatpak remote-ls yields

error: Unable to load summary from remote flathub: Signature made Sat Nov 16 19:28:38 2024 using RSA key ID 562702E9E3ED7EE8
Can't check signature: public key not found

So it seems like we're missing a piece here.

This might(?) be a regression due to !5726 (merged) but really i have no idea (cc @craftyguy )