message: drake: fix an `explain` call
diff: @@ -2085,8 +2085,9 @@ class Builder:
  explain(self, 'the builder hash is invalid')
  execute = True
  if not execute and self._builder_hash != stored_hash:
  - explain(self, 'hash for the builder changed:\n%r\n%r',
  - stored_hash, self._builder_hash)
  + explain(self,
  + 'hash for the builder changed:\n%r\n%r'
  + % (stored_hash, self._builder_hash))
  execute = True
  else:
  explain(self, 'the builder hash is missing')
  