nix: make the project buildable

Previously default.nix could only be used for nix-shell invocation. Now
the deriation can also be built using `nix-build`. The build calls
./test.sh during the checkPhase.

default.nix

@@ -9,9 +9,24 @@ let jekyll_env = bundlerEnv rec {
   };
 in
   stdenv.mkDerivation rec {
+    src = lib.cleanSource ./.;
     name = "jekyll_env";
+
     buildInputs = [ jekyll_env ];
 
+    doCheck = true;
+
+    checkPhase = ''
+      ./test.sh
+    '';
+
+    buildPhase = "true";
+
+    installPhase = ''
+      mkdir $out
+      jekyll build --destination $out
+    '';
+
     shellHook = ''
       jekyll serve --watch
     '';