Can not ping remote builder

[roberth]

Describe the bug

Steps To Reproduce

1. Create a remote builder configuration
2. nix store ping ssh://my-remote-builder
3. SSH asks to authenticate, ignoring the remote builder configuration

Expected behavior

Nix knows how to reach a remote builder when passed via --store.
Simplify(?) the data model such that auth is configured once.

nix-env --version output

Additional context

/etc/nix/machines contains auth credential configuration, referencing a key file

Priorities

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