Fix the assets make target
Fix the assets make target

Presumable, it broke with the introduction of Go modules

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@soundcloud.com>

prometheus/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager