community/ocaml-cppo: remove virtual package from makedepends

replace ocaml-ocamlbuild-dev by the concrete
ocaml-ocamlbuild that provides it.

lua-aports does not see provides in subpackages,
and so will not guarantee that ocaml-ocamlbuild-dev
is built before ocaml-cppo, which sometimes results
in ocaml-cppo being built against an old version of
ocamlbuild; using the concrete package prevents this.

Bump pkgrel to solve the error seen in 7e5b7375
(ocaml-cppo was built against old ocamlbuild for aarch64 and ppc64le)

While here, solve a few more issues with this aport:
  - remove explicit depends for -ocamlbuild subpackage

  - also, remove provides, as this will cause the same
    issue as ocaml-ocamlbuild-dev with build order not
    being guaranteed (no aport depends on ocaml-cppo-dev)

  - add --build-dir=.testenv to `dune runtest` as a
    preventive measure to avoid clobbering _build