shell.nix: add buildifier

Intellij auto-formats BUILD files when buildifier is present.

Change-Id: I07d650ae879e4f7e168cfed698c68ec6df3ef3ae
Reviewed-on: https://review.monogon.dev/c/monogon/+/2082
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/shell.nix b/shell.nix
index b0d4e97..69e6ac4 100644
--- a/shell.nix
+++ b/shell.nix
@@ -34,6 +34,7 @@
   name = "monogon-nix";
   targetPkgs = pkgs: with pkgs; [
     git
+    buildifier
     (stdenv.mkDerivation {
       name = "bazel";
       src = builtins.fetchurl  {