herdr: Use the binary outside Nix to reload the config when package is unset

[sestrella]

Description

If the package is unset, fall back to a herdr binary install outside of Nix's scope.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -A dev --run treefmt.

• Code tested through nix build .#test-all
  or a targeted nix run .#tests -- <pattern>.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News