sideload-repos-systemd: Don't launch service in a tight loop

man systemd.path has the following to say:

> When a service unit triggered by a path unit terminates (regardless
> whether it exited successfully or failed), monitored paths are checked
> immediately again, and the service accordingly restarted instantly.

On my system, I observe that once /run/media/wjt is created,
flatpak-sideload-usb-repo.service is invoked in a tight loop. I think
what's happening is that PathExists=/run/media/wjt continues to be true,
so the service keeps getting restarted.

What we instead want is to activate the .service:

- On login
- When the media directory *changes* (because mount points beneath it
  appear or disappear)

Remove PathExists from the .path, and instead mark the .service as
WantedBy default.target so it is launched on login.

sideload-repos-systemd/flatpak-sideload-usb-repo.path.in

@@ -5,7 +5,6 @@
 # have flatpaks on them, both when a new drive is inserted and at the start of
 # the user session.
 [Path]
-PathExists=@media_dir@/%u
 PathChanged=@media_dir@/%u
 
 [Install]

sideload-repos-systemd/flatpak-sideload-usb-repo.service.in

@@ -3,3 +3,6 @@
 [Service]
 Type=oneshot
 ExecStart=@libexecdir@/flatpak-create-sideload-symlinks.sh @media_dir@/%u
+
+[Install]
+WantedBy=default.target