boards: nrf*bsim: Disable ubsan in bst_install

UBSAN reports a bogus error in this function:
"load of address x with insufficient space for an
object of type '<unknown> *'"
as it seems unable to determine the size of
test_installers.
Let's disable this check in this function.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>