Point GJS_DEBUG_OUTPUT to stderr

Recent gjs changes have made it log most types of messages (including
log() calls) only to a configured log file and not to stderr. So,
set GJS_DEBUG_OUTPUT=stderr to make *stderr* the log file and get
our log() calls back.

src/gnome-shell.in

@@ -152,6 +152,9 @@ def start_shell():
         if os.path.exists(mozjs_libdir + '/libmozjs.so'):
             env['LD_LIBRARY_PATH'] = os.environ.get('LD_LIBRARY_PATH', '') + ':' + mozjs_libdir
 
+    # Log everything to stderr (make stderr our "log file")
+    env['GJS_DEBUG_OUTPUT'] = 'stderr'
+
     if not options.verbose:
         # Unless verbose() is specified, only let gjs show errors and
         # things that are explicitly logged via log() from javascript