main/postmarketos-ui-*: fix lightdm autologin after reinstall (!385)

When the UI package is reinstalled (due to the package being recompiled
or upgraded), it runs $pkgname.post-upgrade instead of
$pkgname.post-install which would run the sed replacement LightDM
autologin. This will cause LightDM to show the greeter when the UI
package is rebuilt, and the user ran pmbootstrap install without
deleting the old chroot.

Signed-off-by: Asriel Dreemurr <asriel.danctnix@gmail.com>
Reviewed-by: Danct12 <danct12@disroot.org>
Signed-off-by: Danct12 <danct12@disroot.org>