pkgbase: Use uname as ABI_FILE

uname is always rebuild on FreeBSD so use this as ABI_FILE for pkg when
building pkg for pkgbase.
pkg uses uname too as default ABI_FILE as of commit d8bbf980b7f6f424fb7cc672c23ab2dfc82b6599
https://github.com/freebsd/pkg/commit/d8bbf980b7f6f424fb7cc672c23ab2dfc82b6599

Discussed with: bapt
MFC after: 1 week