install-packages: Do not prevent login

There's really no reason why *install-packages.service* needs to
complete before users can log in.  Indeed, being able to log in while it
is running may be necessary in order to troubleshoot issues.

install-packages.service

@@ -4,12 +4,11 @@ Description=Install additional packages
 After=network-online.target
 Wants=network-online.target
 Before=zincati.service
-Before=systemd-user-sessions.service
 ConditionPathExists=/etc/ignition/packages.d
 ConditionPathExists=!/etc/ignition/packages.installed
 
 [Service]
-Type=oneshot
+Type=idle
 RemainAfterExit=yes
 ExecStart=/bin/sh /etc/ignition/install-packages.sh
 ExecStartPost=/bin/touch /etc/ignition/packages.installed