[dune] Add to vo rules explicit location of coqlib in boot mode.

When `-coqlib $PATH` is absent, Coq will try to locate coqlib using a
heuristic based on the name of the executable.

The code in `envars.ml` basically does:
```ocaml
Filename.(dirname CUnix.(canonical_path_name (dirname Sys.executable_name)))
```
which doesn't seem to work very well on Windows and OSX when `coqtop`
is a symlink.

Instead, we now pass the right `-coqlib` to coqtop from `coq_dune`.

Maybe fixes #8862.

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