shell.nix: add graphviz

Change-Id: Ic25e11a056a7624ebba923d2b87d947e24c41a20
Signed-off-by: Andrew Dunham <andrew@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f