build: don't copy GOPATH out of docker container

When using GOTOOLCHAIN with make verify the build results copied out of
the dockerized environment contains a go toolchain folder that is
write protected. In order to prevent failures during the cleanup step
opt-out of copying $GOPATH to the host.

build/common.sh

@@ -714,6 +714,7 @@ function kube::build::copy_output() {
     --filter='+ /vendor/' \
     --filter='+ /staging/***/Godeps/**' \
     --filter='+ /_output/dockerized/bin/**' \
+    --filter='- /_output/dockerized/go/**' \
     --filter='+ zz_generated.*' \
     --filter='+ generated.proto' \
     --filter='+ *.pb.go' \