fix build

default.nix

@@ -1,13 +1,19 @@
-{ stdenv, hextra, hugo }: stdenv.mkDerivation
+{ lib, stdenv, hextra, hugo, buildProxy ? null }: stdenv.mkDerivation
 {
   name = "blog";
   src = ./.;
   nativeBuildInputs = [ hugo ];
+  preBuild = lib.optionalString (buildProxy != null) ''source ${buildProxy}'';
   configurePhase =
   ''
     mkdir themes
     ln -s ${hextra} themes/hextra
   '';
-  buildPhase = "hugo";
+  buildPhase =
+  ''
+    runHook preBuild
+    hugo
+    runHook postBuild
+  '';
   installPhase = "cp -r public $out";
 }