dom0-updates: fix reporting when no updates are available

Check `yum check-update` exit code, instead of `grep` - when there are
multiple commands on the single line, $? contains exit code of the last
executed.

Fixes QubesOS/qubes-issues#1475

(cherry picked from commit c46c1e4d2c)

misc/qubes-download-dom0-updates.sh

@@ -82,8 +82,9 @@ fi
 
 if [ "x$PKGLIST" = "x" ]; then
     echo "Checking for dom0 updates..." >&2
-    UPDATES_FULL=`$YUM $OPTS check-update | grep -v "^Loaded plugins:\|^$"`
+    UPDATES_FULL=`$YUM $OPTS check-update`
     check_update_retcode=$?
+    UPDATES_FULL=`echo "$UPDATES_FULL" | grep -v "^Loaded plugins:\|^$"`
     if [ $check_update_retcode -eq 1 ]; then
         # Exit here if yum have reported an error. Exit code 100 isn't an
         # error, it's "updates available" info, so check specifically for exit code 1