verify-boilerplate.sh: add kubernetes-derived files

Signed-off-by: Dr. Stefan Schimanski <stefan.schimanski@gmail.com>

Author: sttts

hack/verify-boilerplate.sh

@@ -25,4 +25,6 @@ echo "Checking file boilerplates…"
 _tools/boilerplate \
   -boilerplates hack/boilerplate \
   -exclude .github \
-  -exclude virtualworkspace/forked_cache_reader.go
+  -exclude virtualworkspace/forked_cache_reader.go \
+  -exclude envtest
+_tools/boilerplate -boilerplates hack/boilerplate/kubernetes -exclude '*' envtest virtualworkspace/forked_cache_reader.go