deploy: Use internal recursive copy rather than libgsystem
deploy: Use internal recursive copy rather than libgsystem

Since we already had a "recursive copy" implementation here, let's
reuse it rather than the libgsystem `gs_shutil_cp_a()`.  Part of the
libglnx porting.

Closes: #428
Approved by: jlebon

GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree