fix #24657: save correct mixer info for instrument change

My fix for InstrumentChange::write() ignores the copy of the Instrument object contained within the InstrumentChange element itself, because that copy does not reflect mixer changes.  Instead, I find the copy of the Instrument contained in the part itself, since this is what the mixer affects, and write that.  Appears to work, although I think at some point we should consider whether storing two copies of the instrument really makes sense, and if so, maybe find a way to keep them in sync.