kubeval: don't build against schema by default

kubeval-schema is a huge 7GB repository that we do not want
to build on hydra. Therefore make it optional.

pkgs/applications/networking/cluster/kubeval/default.nix

···
 { stdenv, lib, fetchFromGitHub, buildGoModule, makeWrapper }:
 
-let
-
-  # Cache schema as a package so network calls are not
-  # necessary at runtime, allowing use in package builds
-  schema = stdenv.mkDerivation {
-    name = "kubeval-schema";
-    src = fetchFromGitHub {
-      owner = "instrumenta";
-      repo = "kubernetes-json-schema";
-      rev = "6a498a60dc68c5f6a1cc248f94b5cd1e7241d699";
-      sha256 = "1y9m2ma3n4h7sf2lg788vjw6pkfyi0fa7gzc870faqv326n6x2jr";
-    };
-
-    installPhase = ''
-      mkdir -p $out/kubernetes-json-schema/master
-      cp -R . $out/kubernetes-json-schema/master
-    '';
-   };
-
-in
-
 buildGoModule rec {
   pname = "kubeval";
   version = "0.14.0";
···
     sha256 = "0kpwk7bv36m3i8vavm1pqc8l611c6l9qbagcc64v6r85qig4w5xv";
   };
 
-  buildInputs = [ makeWrapper ];
-
   modSha256 = "0y9x44y3bchi8xg0a6jmp2rmi8dybkl6qlywb6nj1viab1s8dd4y";
-
-  postFixup = "wrapProgram $out/bin/kubeval --set KUBEVAL_SCHEMA_LOCATION file:///${schema}/kubernetes-json-schema/master";
 
   meta = with lib; {
     description = "Validate your Kubernetes configuration files";

pkgs/applications/networking/cluster/kubeval/schema.nix

···
+{ fetchFromGitHub }:
+# To cache schema as a package so network calls are not
+# necessary at runtime, allowing use in package builds you can use the following:
+
+#   KUBEVAL_SCHEMA_LOCATION="file:///${kubeval-schema}";
+(fetchFromGitHub {
+  name = "kubeval-schema";
+  owner = "instrumenta";
+  repo = "kubernetes-json-schema";
+  rev = "6a498a60dc68c5f6a1cc248f94b5cd1e7241d699";
+  sha256 = "1y9m2ma3n4h7sf2lg788vjw6pkfyi0fa7gzc870faqv326n6x2jr";
+}) // {
+  # the schema is huge (> 7GB), we don't get any benefit from building int on hydra
+  meta.hydraPlatforms = [];
+}

pkgs/top-level/all-packages.nix

···
 
   kubeval = callPackage ../applications/networking/cluster/kubeval { };
 
+  kubeval-schema = callPackage ../applications/networking/cluster/kubeval/schema.nix { };
+
   kubernetes = callPackage ../applications/networking/cluster/kubernetes {
     go = buildPackages.go_1_13;
   };