Reenable metrics for OCaml 5.2

[Firobe]

Use a version of mirage-memtrace compatible with OCaml 5.2 (mirroring similar changes on janestreet/memtrace#22). The underlying statmemprof isn't available before 5.3, but this allows us to at least collect all the other stats in mirage-monitoring (including GC).

Now we can use the original Dockerfile, and just add pins for ocaml-solo5 and memtrace-mirage. This also allows passing the same flags at runtime.