build-devtoolbox: Work around broken SELinux in toolbox containers

Install selinux-policy first and remove all of its files, to force
SELinux to be inert in the container. This unbreaks the subsequent `dnf
builddep cockpit` which drags in selinux-policy as a dependency, causes
long hangs, tons of lsetfilecon errors, and eventually lots of package
install failures.

See https://bugzilla.redhat.com/show_bug.cgi?id=1768075