[test/tox] Enforce installation of modified yapps 580a6821713d is not enough for tox apparently... The thing is that it will first install dependencies (the `deps` option from tox.ini), actually pulling yapps from Logilab github but then, it will install the project (that is, CubicWeb) from the sdist *along with its dependencies*, which will pull yapps again and override the previously installed version for some mysterious reason. So add a `pip install` command in tox.ini, so that the modified yapps is the last one installed.

[web] delete unused PropertySheet._sourcefile_mtime

[etwist] remove no-longer-necessary uicache cleaning It was done before we were actually up, which means it could be too eager when combined with #6893185. Closes #6893247.