Make generated build tag parameterizable for go2idl

Downstream generators that want to reuse the upstream generated types
need to be able to define a different ignore tag (so that they can see
the already generated types).

hack/verify-flags/known-flags.txt

@@ -37,6 +37,7 @@ bind-pods-burst
 bind-pods-qps
 build-only
 build-services
+build-tag
 cadvisor-port
 cert-dir
 certificate-authority