nixos/geoipupdate: Improve secret handling
nixos/geoipupdate: Improve secret handling

Make secret replacement more robust and futureproof:

- Allow any attribute in `services.geoipupdate.settings` to be a
  secret if set to `{ _secret = "/path/to/secret"; }`.

- Hash the license key path before using it as a placeholder in the
  config file to minimize the risk of conflicting file paths being
  replaced instead.

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