prevent unsynchronized output changes access

The raw plan output changes were stored in the output exec node, when
they should have instead been fetch lazily through the context via the
synchronized ChangesSync value.

internal/plans/changes_sync.go

@@ -191,6 +191,7 @@ func (cs *ChangesSync) RemoveOutputChange(addr addrs.AbsOutputValue) {
 	defer cs.lock.Unlock()
 
 	addrStr := addr.String()
+
 	for i, o := range cs.changes.Outputs {
 		if o.Addr.String() != addrStr {
 			continue