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)

nixos/modules/services/web-servers/lighttpd/gitweb.nix

···
               "/gitweb/"        => "${pkgs.git}/share/gitweb/gitweb.cgi"
           )
           setenv.add-environment = (
-              "GITWEB_CONFIG" => "${gitwebConfigFile}"
+              "GITWEB_CONFIG" => "${gitwebConfigFile}",
+              "HOME" => "${cfg.projectroot}"
           )
       }
     '';