systemd-ask-password-plymouth.service.in: Do not hardcode /run path to pid file

Use @plymouthruntimedir@/pid in systemd-ask-password-plymouth.service.in so
that the way the path to the pidfile is build there is identical to the
way the --pidfile argument is build in plymouth-start.service.in.

Fixes #26

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

systemd-units/systemd-ask-password-plymouth.service.in

@@ -7,7 +7,7 @@ After=plymouth-start.service
 Before=shutdown.target
 ConditionKernelCommandLine=!plymouth.enable=0
 ConditionVirtualization=!container
-ConditionPathExists=/run/plymouth/pid
+ConditionPathExists=@plymouthruntimedir@/pid
 
 [Service]
 ExecStart=@SYSTEMD_ASK_PASSWORD_AGENT@ --watch --plymouth