[test] do not overwrite user's CW_CUBES_PATH in unittest_migration
Currently when you rely on your CW_CUBES_PATH environment variable to
locate cubes necessary for cubicweb's tests, the migration test crashes
because it overrides the env var.
make sync_schema_props_perms(<computed rtype>) work as expected
It currently ends up with an ExecutionError while we want to synchronize
permissions. This makes 3.21.1 migration of any application using computed
relation crashing.
Also, remove pre 3.6 compat code in hooks that prevent living schema update on
computed relation's permission changes.