assistant: Let users pick the OS from a list if it is unknown

When we fail to detect an operating system with libosinfo, we often
go with our default settings. Those work for the majority of OSes
but not for all. By allowing users to enter which operating system
they intend to install, we can cover many more installations by
offering better support for unrecognized medias.
2 jobs for tell-us-what-os-this-is in 80 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #835112
flatpak
flatpak

01:14:46

 
  Fedora
passed #835113
fedora:rawhide

00:05:36