builds: Temporarily switch nightly Bulgarian voice back to 'espeak'

Turns out the 'gtts' tool on the buildserver is too old to support
Bulgarian.  Once this is dealt with, go ahead and switch back.

Change-Id: I507b3b736a722569a2050a2e1f61ed4dce210a44

tools/builds.pm

@@ -610,10 +610,11 @@ sub simbuilds {
 	'lang' => 'bulgarian',
 	'name' => 'Български (Bulgarian)',
         'short' => 'bg',
-        'defengine' => 'gtts',
+        'defengine' => 'espeak',  # XXX Switch to gtts when buildserver is updated
 	'engines' => {
 	    'espeak' => '-vbg',
 	    'gtts' => '-l bg',
+            # No piper voice yet.
 	},
         'enabled' => 1,
     },