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.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>