go_1_9: mark as insecure (see the parent commit)

Upstream only supports two latest releases.
golang/go#29903 (comment)

(cherry picked from commit a687ef9)

pkgs/development/compilers/go/1.9.nix

@@ -180,6 +180,7 @@ stdenv.mkDerivation rec {
   disallowedReferences = [ go_bootstrap ];
 
   meta = with stdenv.lib; {
+    knownVulnerabilities = [ "CVE-2019-6486" ];
     branch = "1.9";
     homepage = http://golang.org/;
     description = "The Go Programming language";