go-mod.bbclass: use append to add `modcacherw`

This class provided default build flags but includes the base go class
which also provides default flags.  Use a different mechanism to
extend the default flags without discarding `-trimpath` from the base
class.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>