liveinst: Ensure DBUS_SESSION_BUS_ADDRESS is set in live install

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

pkexec more thoroughly cleans the environment before elevating
privileges than consolehelper so DBUS_SESSION_BUS_ADDRESS gets
expunged.

Anaconda needs that variable to disable the screensaver (and
for other reasons).

This commit ensures it gets set explicitly after privileges have
been raised.

data/liveinst/liveinst

@@ -26,6 +26,11 @@ if [ "$(id -u)" -ne 0 ]; then
     exec pkexec "$0" "$@"
 fi
 
+# pkexec clears DBUS_SESSION_BUS_ADDRESS from environment
+if [ -z "$DBUS_SESSION_BUS_ADDRESS" ]; then
+    export DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/${PKEXEC_UID}/bus
+fi
+
 # Allow running another command in the place of anaconda, but in this same
 # environment.  This allows storage testing to make use of all the module
 # loading and lvm control in this file, too.