`preprocess` `per_module` dependencies are not relative

Expected Behavior

When using preprocess the binary in an run action should be part of

(library
    ...
    (preprocess  (per_module
                  ((action (run ./gen/gen.exe %{input-file})) myModule))))

Should depend on ./gen/gen.exe and build myModule.

Actual Behavior

Preprocessing with ./gen/gen.exe in rule but in preprocess stanzas, it can't find the rule to build gen.exe because it expects ./gen/gen.exe live relative to the workspace and not relative to the dune file that contains the preprocess stanza:

$ dune build
  File "src/.merlin-conf/_unknown_", line 1, characters 0-0:
  Error: No rule found for gen/gen.exe
  [1]

Reproduction

• PR with a reproducing test: https://github.com/ocaml/dune/pull/7463

Specifications

• Version of dune (output of dune --version): current main
• Version of ocaml (output of ocamlc --version): 4.14.1
• Operating system (distribution and version): Fedora GNU/Linux

Start with the reproducing test in PR #7463 and the Dune contributor guidance in doc/hacking.rst. Reproduce the preprocess/per_module case on current main, then trace how ./gen/gen.exe is resolved relative to the Dune file; done means the dependency is found and myModule builds successfully.