ci: Ignore main file in licenseok

This should fix test failures that were being caused by golang/go#24570,
which caused test binaries to be built even when there are no test
files. The poor interaction there arose from the setting of flags in our
init() function, it seems.

Makefile

@@ -15,7 +15,7 @@ build:
 	cp ./release/dep-$(PLATFORM)-$(ARCH) dep
 
 licenseok:
-	go build ./hack/licenseok
+	go build -o licenseok ./hack/licenseok/main.go
 
 validate: build licenseok
 	./hack/lint.bash

hack/licenseok/main.go

@@ -1,3 +1,5 @@
+// +build ignore
+
 // Copyright 2017 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

hack/validate-licence.bash

@@ -7,6 +7,6 @@
 # source files to check licence
 set -e
 
-go build ./hack/licenseok
+go build -o licenseok ./hack/licenseok/main.go
 find . -path ./vendor -prune -o -regex ".+\.pb\.go$" -prune -o -type f -regex ".*\.\(go\|proto\)$"\
  -printf '%P\n' | xargs ./licenseok