Fix cancel action on deletion confirmation form
Currently crashed with RequestError: invalid action '__action_cancel'.
It's broken as of https://hg.logilab.org/master/cubicweb/rev/cb217b2b3
This fix is similar as the one done in
https://hg.logilab.org/master/cubicweb/rev/9f1c89e7426d
Closes #13521493
[rql2sql] Test and fix some case of sql generation
when an unstable variable (ie whose type change from one solution to another) is
used in an EXISTS node but belongs to an outer scope.
Closes #13518646.