nightly: blacklist gateway-dk

The board's image build needs the gateway-dk-ask extension to
produce a usable artifact; the stock nightly pipeline doesn't
exercise that path. Skip it from nightly until the gateway-dk-ask
build flow is wired in here.

Author: igorpecovnik

release-targets/targets-release-nightly.blacklist

@@ -24,3 +24,4 @@ mekotronics-r58-4x4
 mekotronics-r58hd
 dg-svr-865-tiny
 sk-am64b
+gateway-dk