virtualbox: give full url for downloading the ext (#15869)

The user only has to agree on the terms and conditions before
downloading the file. We might as well give him access to the full URL
by default.
(cherry picked from commit a6593a16f7aa1de03ed0d4f36e8bfb68c0459df5)

pkgs/applications/virtualization/virtualbox/default.nix

···
     sha256 = "11f40842a56ebb17da1bbc82a21543e66108a5330ebd54ded68038a990aa071b";
     message = ''
       In order to use the extension pack, you need to comply with the VirtualBox Personal Use
-      and Evaluation License (PUEL) by downloading the related binaries from:
+      and Evaluation License (PUEL) available at:
 
-      https://www.virtualbox.org/wiki/Downloads
+      https://www.virtualbox.org/wiki/VirtualBox_PUEL
 
-      Once you have downloaded the file, please use the following command and re-run the
-      installation:
+      Once you have read and if you agree with the license, please use the
+      following command and re-run the installation:
 
-      nix-prefetch-url file://${name}
+      nix-prefetch-url http://download.virtualbox.org/virtualbox/${version}/${name}
     '';
   };