Improve use of sops-nix

I did an experiment combining sops-nix and LoadCredential. See details at the top of https://github.com/haskellfoundation/hf-infrastructure/blob/b2f63b4d6d1e3a532761c242283e3ed8aec11998/stackage-builder/nixos-modules/stackage-server.nix .

In those details, I highlight a shortcoming of the current approach. Now I think I could do a little better:

* [ ] Use `builitns.dirOf` to get the directory where secrets are held

Start at the experiment details near the top of stackage-builder/nixos-modules/stackage-server.nix and review how the current secrets directory is obtained. Use builtins.dirOf as described in the checklist, and consider the work done when the configuration derives the directory containing the secrets.

Written by the indexing model from the issue text.