sideload-repos-systemd: Use systemd-tmpfiles

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.

sideload-repos-systemd/Makefile.am.inc

@@ -8,11 +8,13 @@ systemduserunit_DATA += sideload-repos-systemd/flatpak-sideload-usb-repo.service
 path_in_files = sideload-repos-systemd/flatpak-sideload-usb-repo.path.in
 systemduserunit_DATA += sideload-repos-systemd/flatpak-sideload-usb-repo.path
 
-systemdsystemunit_DATA += sideload-repos-systemd/flatpak-sideload-repos-dir.service
-
 %.path: %.path.in config.log
  $(AM_V_GEN) $(SED) -e "s|\@media_dir\@|$(RUN_MEDIA_DIR)|" $< > $@
 
+tmpfilesdir = $(prefix)/lib/tmpfiles.d
+dist_tmpfiles_DATA = sideload-repos-systemd/tmpfiles.d/flatpak-sideload-repos.conf
+
 endif
 
-EXTRA_DIST += sideload-repos-systemd/flatpak-sideload-usb-repo.path.in sideload-repos-systemd/flatpak-sideload-usb-repo.service.in sideload-repos-systemd/flatpak-sideload-repos-dir.service
+EXTRA_DIST += sideload-repos-systemd/flatpak-sideload-usb-repo.path.in sideload-repos-systemd/flatpak-sideload-usb-repo.service.in
+

sideload-repos-systemd/tmpfiles.d/flatpak-sideload-repos.conf

@@ -0,0 +1 @@
+D /run/flatpak/sideload-repos 0777 - - -