Always build config.ml in the current opam switch

[samoht]

This is currently the case if config.ml is at the root of the dune-workspace, as build.config contains:

(data_only_dirs universe)
...

But this is not true, if someone does call opam-monorepo lock manually from a different directory (like https://github.com/mirage/mirage-skeleton/blob/mirage-dev/Makefile#L64) to build a larger monorepo (here to be able to build all the configured unikernels in one go).

We should probably move this in dune-workspace.config - or create a workspace for config and make sure the duniverse directory is ignored in that worskpace.

[TheLortex]

We should probably move this in dune-workspace.config - or create a workspace for config and make sure the duniverse directory is ignored in that worskpace.

There's a lot of scaffolding needed to be able to ignore any duniverse folder in the workspace. I'm not even sure that's possible yet.

For that matter I'm thinking about not using dune to build the config.ml. The file being simple enough, it can be executed as a script, simply using ocaml config.ml. By adding two directives before the beginning of the file, we can execute configurations as is:

#use "topfind"
#require "mirage"

EDIT:
It's not that easy. We want config.ml to be linked with the library that was linked with the mirage executable in scope. It might be in the opam switch, but not necessarily (for example in tests).