How to access the ocaml version string within a dune file?

I have this in a src/dune file:

      ((action (run cppo -V OCAML:`ocamlc -version` %{input-file})) RNG)))

Of course, it does not work.
So, is there already a string or env. var. defined with the result of the shell command ocamlc -version?

answer to self:

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