Buggy `enabled_if` for executable

[toots]

The following gist: https://gist.github.com/toots/fa72f3a6038ab2b3b3668e242e2499e8 has dune language at 3.23 and uses a read directive for enabled_if on an executable.

This results in the following bogus error:

$ dune build                                                                                                                                                                            14:22:25
File "dune", line 3, characters 16-29:
3 |  (enabled_if (= %{read:./foo} true)))
                    ^^^^^^^^^^^^^
Error: Only architecture, system, model, os_type, ccomp_type, profile,
ocaml_version, context_name, arch_sixtyfour and env variables are allowed in
this 'enabled_if' field. Please upgrade your dune language to at least 3.15.