dune cannot execute pkg build rules with `dune` when not in PATH

[Alizter]

Reproduced here:
https://github.com/Alizter/dune/actions/runs/19168554044/job/54795067158

The current lookup for dune relies on PATH containing the dune binary. This is wrong for a few reasons, firstly we might not have the same dune binary in PATH and secondly it might not even be there like here:

File "dune.lock/sexplib0.pkg", line 4, characters 6-10:
4 |  (run dune build -p %{pkg-self:name} -j %{jobs}))
          ^^^^
Error: Program dune not found in the tree or in PATH
 (context: default)

[Alizter]

Reproduction case in #12698. This is important to fix as it means we cannot run dune without it being correctly found in PATH, which is absurd.

[rgrinberg]

This is close enough to #10019 that I'm just marking this as a dupe.