Remove old build tag comments

Go is transitioning from the old '// +build' form to '//go:build', this
removes all uses of the old form, adding the new one where needed.

See https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

cmd/osbuild-auth-tests/main_test.go

@@ -1,5 +1,4 @@
 //go:build integration
-// +build integration
 
 package main