Do not litter EduMIPS64/web.edumips.org repo on pushing new web UI versions

Currently there is a lot of unneeded files in the root of the EduMIPS64/web.edumips.org repository, caused by the careless pushing of newly built Web UI files by the Github Action pipeline without cleaning up first.

We should identify a way to push files cleanly, keeping only the ones that are needed to run the current version of the web UI.