Patch graphviz in emacs-guix

[jgart]

Hi should emacs-guix be patched and the graphviz program add as an input to 
emacs-guix?

https://github.com/alezost/guix.el/blob/c9aef52121b458297e70bb50f49f7276b4a8d759/elisp/guix-external.el#L51

If I try to run `guix-package-graph` I get the following error:

guix-dot-arguments: Couldn’t find ’dot’.
Set ‘guix-dot-program’ to a proper value