[build] Upgrade dune to 2.5 (with cache fix for MacOS)

Summary:
Release notes [are here](https://github.com/ocaml/dune/releases/tag/2.5.0).

Among othe things this release includes my fix for dune freezing on
MacOS when cache is enabled. So now, switching between `make byte`,
`make infer`, and `make test_build` can be super fast.

Documentation about dune caching [is here](https://dune.readthedocs.io/en/stable/caching.html).

Reviewed By: jvillard

Differential Revision: D21016684

fbshipit-source-id: 7208dddd2

opam.locked

@@ -54,7 +54,7 @@ depends: [
   "core_kernel" {= "v0.13.0"}
   "cppo" {= "1.6.6"}
   "ctypes" {= "0.16.0"}
-  "dune" {= "2.3.1"}
+  "dune" {= "2.5.0"}
   "dune-configurator" {= "2.3.1"}
   "dune-private-libs" {= "2.3.1"}
   "easy-format" {= "1.3.2"}