allow dune exec -w + dune build -w running at the same time

[mudrz]

Desired Behavior

From https://discuss.ocaml.org/t/dune-exec-dune-build-running-in-the-same-project-web-server-melange/13409

A full stack OCaml project with melange has a server and a react app:

dune-project
  my_server
  my_react_app

It would be nice to start the web server with dune exec -w my_server
and build the melange project with dune build -w @my_react_app.
Currently you get a dune lock error: A running dune (pid: ...) instance has locked the build directory.

The suggestions in the topic were:

• DUNE_CONFIG__GLOBAL_LOCK=disabled , which works but it's not documented and unstable
• running dune exec with --no-build, which works if -w is not used, but results in the same lock error as before when -w is used

cc @Alizter

[jchavarri]

Potential duplicate of #7110. Related to #8114.