ci-buildpackage: Use a less common name for the build directory

Building packages into _build causes issues with ocaml packages: those too build into this directory by default, which is therefore removed during the clean stage.

This has been observed for ounit, pcre-ocaml and ocaml-result, and probably affects all ocaml packages.

Using a less common name for the build directory solves the issue without having to patch individual packages.