/lib/modules/<version>/build is symlinked to /usr/src/<version>, and since lbuild extracts the rpm to put in 'reused' directory, it has to take care of the symlink too to prevent requiring the rpm to be installed. Otherwise we see some errors like this

error: cannot determine ksym provides of /tmp/rpmbuild-lustre-jenkins-LkKoPYLF/BUILDROOT/lustre-2.15.51_9_g8ab411e-1.x86_64/lib/modules/4.12.14-122.116-default/updates/lustre-client-tests/fs/llog_test.ko - missing ksym-povides tool.