[rewriter] fix latent bug: arbitrary etype may be substituted when using is_instance_of type restriction Note: the test is not deterministic because the bug depends on the iteration order on a set.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl