release: Only check no-root/no-qemu when building

Building VMs as non-root requires no-QEMU code paths (installing
packages from outside the VM image rather than inside it) and vice
versa; we have a check for broken combinations.

Unfortunately that check was breaking
make -C src/usr.sbin/pkg NO_ROOT=YES -V PKGCONFBRANCH
because that code reaches into src/release to determine the branch
name (which is then used to determine which /etc/pkg/FreeBSD.conf to
install).

Wrap the no-root/no-qemu check in an .if to only run when we've
asked for VM and/or CLOUD building to be enabled.

Reviewed by: ivy
MFC after: 5 minutes
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D53486

(cherry picked from commit b485322d6b5846fe4fb5aff6c4a07221b09946fc)