add: hci eval (or repl) for local debugging

[adrian-gierakowski]

Since herculesCI is a function, users are not able to (easily) evaluate and verify what happens within it on their local machines. hci eval could to the same as hci run in regards to providing the config which is present in CI but instead of running an effect it could allow to eval any path within the attrset returned by herculesCI. Alternatively, or ideally additionally, hci repl could drop a user into nix repl with herculesCI loaded as top level variable.

[roberth]

• hci effect eval Add hci effect eval #661
  • prints derivation path
• repl
• hci eval - eval any path within herculesCI