message: Explain builder hash differences.
diff: @@ -2085,7 +2085,8 @@ 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  - explain(self, 'hash for the builder is outdated')
  + 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')
  