webui: Handle XAUTHORITY and XDG_RUNTIME_DIR

The titlebar with "Mozilla Firefox" has been fixed but there
are some more bits that can be cleaned up.

This commit achieves that by:

1. Make sure more of the environment is bubbled through anaconda to
   the webui launcher. In particular, we need XDG_CURRENT_DESKTOP, but
   this commit brings it all through, so firefox runs in an environment
   as close to getting run directly by the live user as possible.
2. Two exceptions are XAUTHORITY and XDG_RUNTIME_DIR which need to
   remain unset until we can run firefox as a normal user instead of root.

data/liveinst/liveinst

@@ -24,13 +24,17 @@ BACKEND_READY_FLAG=/run/anaconda/backend_ready
 # unprivileged, restart running as root.
 if [ "$(id -u)" -ne 0 ]; then
     xhost +si:localuser:root
-    unset XAUTHORITY
     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
+# pkexec clears the environment, so get it back
+if [ -n "${PKEXEC_UID}" ]; then
+    INSTALLER_USER=$(id -n -u "${PKEXEC_UID}")
+    readarray -t user_environment < <(pkexec --user "${INSTALLER_USER}" env XDG_RUNTIME_DIR="/run/user/${PKEXEC_UID}" systemctl --user show-environment)
+
+    for variable in "${user_environment[@]}"; do
+      export "${variable?}"
+    done
 fi
 
 # Allow running another command in the place of anaconda, but in this same