Instead of using a systemd service to create the directory we need for
sideloading (/run/flatpak/sideload-repos), use a systemd-tmpfiles conf
file. This is more elegant, and the service also had a bug which meant
it would create a "/run/flatpak;" directory instead of the one intended.
Since systemd-tmpfiles-setup.service runs before sysinit.target and path
units are run after sysinit.target, the tmp directory should be in place
when flatpak-sideload-usb-repo.path runs in the systemd user instance.
Endless OS already has a file at /lib/tmpfiles.d/flatpak.conf, so we
can't use that filename.
