release: Fix pkgbase tarball names on FTP site

These need to have names like

FreeBSD-15.0-ALPHA5-amd64-pkgbase-repo.tar

not just "pkgbase-repo.tar".

Fixes: d0ff67db2908 ("release: Publish pkgbase-repo.tar")
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva