link the packpath in expected folder so that even unwrapped neovim can pick home-manager's plugins.
I sometimes need to run neovim not wrapped/configured by nix (when
developing neovim or when other projects bring their own neovim in
PATH). Currently they dont find plugins installed by home-manager in the
cases where packpath is not set to the generated nix packpath directory.
With this change, neovim can discover HM-installed plugins by itself.
deb2f59b5c