fix trivial shell quoting issues surfaced by shellcheck v0.7.1

hack/lib/golang.sh

@@ -874,7 +874,7 @@ kube::golang::build_binaries() {
         cat "/tmp//${platform//\//_}.build"
       done
 
-      exit ${fails}
+      exit "${fails}"
     else
       for platform in "${platforms[@]}"; do
         kube::log::status "Building go targets for ${platform}:" "${targets[@]}"