screensaver: Look at $PKEXEC_UID not $USERHELPER_UID

Since commit 919be1d anaconda has
been using pkexec instead of consolehelper to get root privileges
on the live image.

This caused a regression on the KDE live image, because anaconda
was trying to inhibit the screensaver as the $USERHELPER_UID user
which no longer gets set.

This commit changes the code to use $PKEXEC_UID which is the polkit
equivalent.

pyanaconda/screensaver.py

@@ -63,15 +63,15 @@
 class SetEuidFromConsolehelper():
     """Context manager to temporarily set euid from env. variable set by consolehelper.
 
-    Live installs use consolehelper to run as root, which sets the original UID in $USERHELPER_UID.
+    Live installs use pkexec to run as root, which sets the original UID in $PKEXEC_UID.
     """
     def __init__(self):
         self.old_euid = None
 
     def __enter__(self):
-        if "USERHELPER_UID" in os.environ:
+        if "PKEXEC_UID" in os.environ:
             self.old_euid = os.geteuid()
-            new_euid = int(os.environ["USERHELPER_UID"])
+            new_euid = int(os.environ["PKEXEC_UID"])
             os.seteuid(new_euid)
         return self