Fix stage-qa problems with devel/cmake

Linking to base libarchive is disallowed by stage-qa, so restore
USES=libarchive (to avoid the bundled libarchive and to link to
ports libarchive) and drop the pkg(8) generator for CPack, since
libpkg in turn pulls in base libarchive.

PR: 227372
Approved by: tcberner (mentor, implicit)