Hi,

> +    (arguments
> +     `(#:phases
> +       (modify-phases %standard-phases
> +         (add-after 'unpack 'fix-sleep-path
> +           (lambda* (#:key inputs #:allow-other-keys)
> +             (let ((coreutils (assoc-ref inputs "coreutils")))

coreutils is in native-inputs, not inputs, so this needs to be:
(assoc-ref (or native-inputs inputs) "coreutils").  (native-inputs
and inputs are appended together when compiling natively, that's
why this didn't cause a build error).

> +               (substitute* "./test/startup_failure.py"
> +                 (("sleep") (string-append coreutils "/bin/sleep")))))))))

You can write (("sleep") (which "sleep")) here.

> +    (native-inputs
> +     `(("pkg-config" ,pkg-config)
> +       ("scdoc" ,scdoc)
> +       ;; For tests
> +       ("python" ,python)
> +       ("coreutils" ,coreutils)))

FWIW, coreutils is an implicit native-input of meson-build-system,
so this input isn't necessary.

Greetings,
Maxime.