release: Add NODISTSETS to skip legacy dist sets

make -DPKGBASE -DNODISTSETS can now be used to build an installer
without any support for dist sets.

Reviewed by: Isaac Freund <ifreund@freebsdfoundation.org>
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50919