set process in jade filter every setup()

as per https://github.com/miracle2k/webassets/blob/master/src/webassets/filter/__init__.py#L246:

def setup(self):
...
"Note: This may be called multiple times if one filter instance is used with different asset environment instances."
...

If setup is called multiple times, this change means the Jade Filter object will have the same arguments to run every time.

src/webassets/filter/jade.py

@@ -73,7 +73,7 @@ def setup(self):
         """
         super(Jade, self).setup()
 
-        self.argv.append(self.jade or 'jade')
+        self.argv = [self.jade or 'jade']
         self.argv.append('--client')
 
         if self.jade_no_debug: