As we discussed this possibility with @mikem today on CentOS Dojo:

There is an option in mock called "--bootstrap-chroot", but it is not always needed and/or desired.
Now it is handled by a mock which does nesting, but that doesn't always work. For instance, if Python rebase happens, DNF is not installable
for a while.

Proposal:

• Create a build-tag (f30-system-build)
• Configure build target (f30-build) to use f30-system-build for buildroot preparation
• Tag into f30-system-build PM-related builds (rpm, dnf, …) when necessary
• Whenever koji prepares buildroot:
  • Use PM from builder to make "bootstrap" chroot (which contains dnf/rpm/builddep)
  • (possibly?) Optimize this by having a tag in SRPM that will indicate features required and koji will have to check whether those features provided by builder's PM, and if it does, do not use "bootstrap" chroot
• Use that "bootstrap" chroot to prepare real buildroot for a packages