nixos/lighttpd: set $HOME for gitweb sub-service
This allows gitweb to expand '~' in /etc/gitconfig. Without a $HOME
variable, it fails to list any projects and instead show the text
"No such projects found" in the UI.
Setting $HOME to the gitweb project root seems like a sensible value.
(cherry picked from commit d916ce2ef40e412c361805531eff5966a309da1d)
