* docs: Use secret-key-files when demonstrating post-build-hooks
The docs used to recommend calling `nix store sign` in a post-build
hook, but on more recent versions of nix, this results in unsigned
store paths being copied into binary caches. See
https://github.com/NixOS/nix/issues/6960 for details.
Instead, use the `secret-key-files` config option, which signs all
locally-built derivations with the private key.
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
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