Fix (org +present) by downloading reveal.js

See the following commit in doom-emacs:
> commit 723583e8d737fac93a05fe9dc06a233f3efdc305
> Author: Diego Zamboni <diego@zzamboni.org>
> Date:   Thu Aug 27 07:15:14 2020 +0200
>
>     Automatically download reveal.js and use local copy, default to v4 (#3846)
>
>     * Automatically download reveal.js upgrade to v4
>
>     The +present option now automatically checks out the reveal.js
>     repository and configures org-re-reveal to use it. It also now uses
>     reveal.js 4 instead of 3.9.2.
>
>     * Changed :init to :config

overrides.nix

@@ -71,6 +71,18 @@ self: super: {
     pname = "php-extras";
   };
 
+  revealjs = self.straightBuild {
+    pname = "reveal.js";
+    ename = "revealjs";
+
+    installPhase = ''
+      LISPDIR=$out/share/emacs/site-lisp
+      install -d $LISPDIR
+
+      cp -r * $LISPDIR
+    '';
+  };
+
   rotate-text = self.straightBuild {
     pname = "rotate-text.el";
     ename = "rotate-text";