main/postmarketos-ui-*: fix for lightdm autologin when package is reinstalled.

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.