Upgrade to (lang dune 3.0)

[kit-ty-kate]

According to @rgrinberg:

Older dune doesn’t have proper support for directory targets, so stale artifact deletion would run and delete the contents of the directory

This is the case for make reftest-gen (aka. dune build @reftest-gen) which starts by deleting every opam roots and opam repositories in _build/default/tests/reftests previously created by make test, which makes that command take upwards of 70 seconds before actually running reftest-gen.
EDIT: Actually this is a separate issue fixed in #6155

Currently this PR does not work and makes every tests fail with:

[WARNING] No switch is currently set, perhaps you meant '--set-default'?
Fatal error:
/<path>/_build/default/src/client/opamMain.exe: "open" failed on
/tmp/build_ff9d44_dune/opam-reftest-7c8979/OPAM/repo/lock: Permission denied

and i have no idea why. Does anyone have an idea?

[kit-ty-kate]

The original issue was fixed in #6155 so closing this. We'll see about the permission issue whenever we upgrade to (lang dune 3.0)