roachpb: clone Transaction in Error.UpdateTxn

I'm not aware of this causing issues, but we're pretty careful with this
in other places (like BatchResponse_Header.combine) to avoid issues like
what we saw in cockroachdb#34241, so this seems like a good change.

Release note: None

pkg/roachpb/errors.go

@@ -193,7 +193,7 @@ func (e *Error) UpdateTxn(o *Transaction) {
 		return
 	}
 	if e.UnexposedTxn == nil {
-		e.UnexposedTxn = o
+		e.UnexposedTxn = o.Clone()
 	} else {
 		e.UnexposedTxn.Update(o)
 	}