packaging: fix "make centos{5,6,7}"
Without this change, make centos{5,6,7} fails with error:
cp: cannot stat '<basedir>/rpmbuild/RPMS/*/*': No such file or directory
This change is the exact equivalent of
bc4bbc42899a (which applied to Fedora).
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}