fix use of nspawn https://github.com/rpm-software-management/mock/pull/679

Mock v2.7

Release notes: https://github.com/rpm-software-management/mock/wiki/Release-Notes-2.7

• bootstrap: copy-in katello CA pem file if exists
• early error when bootstrap is off and external buildrequires are detected (msuchy@redhat.com)
• hotfix preexec_fn traceback on RHEL 8 s390x (issue 653)
• introduce external buildrequires (msuchy@redhat.com)
• add rpkg spec preprocessing capability (clime@fedoraproject.org)
• sign plugin: don't ignore signing command failure
• don't setsid() twice with --shell
• better logging when dynamic BR detected (msuchy@redhat.com)
• do not TB if rpmbuild fails with exit code 11 (msuchy@redhat.com)
• fix addrepo when repo is missing (markus.linnala@gmail.com)
• own the cheat directory
• Allow percent-sign in config_opts['resultdir']
• add a new "postupdate" hook (dturecek@redhat.com)
• log mock's NVR