another approach that worked: use --noclean to make rpmbuild not hit the error, and clean up afterwards with rm (it's usually a script running rpmbuild anyway)
i'm moving a software product from rhel7.x to rhel8 just this week. so i started with 8.2 because that was the latest. i don't have a %clean in my .spec, but i get similar errors:
Wrote: ./<snip>-5.2.428.g3a3f2aee-1.el8.x86_64.rpm
Executing(%clean): /bin/sh -e /var/tmp/rpm-tmp.ALKfqp
+ umask 022...