helmfile: add helm to path with wrapProgram

Helmfile makes direct calls to `helm` as an implicit dependency. Now helmfile
can work without Helm also being explicitly installed.

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

···
-{ lib, buildGoPackage, fetchFromGitHub, ... }:
+{ lib, buildGoPackage, fetchFromGitHub, makeWrapper, kubernetes-helm, ... }:
 
 let version = "0.19.0"; in
 
···
   };
 
   goPackagePath = "github.com/roboll/helmfile";
+
+  nativeBuildInputs = [ makeWrapper ];
+
+  postInstall = ''
+    wrapProgram $bin/bin/helmfile \
+      --prefix PATH : ${lib.makeBinPath [ kubernetes-helm ]}
+  '';
+
 
   meta = {
     description = "Deploy Kubernetes Helm charts";