tmp.kiwix.org should not be used to store kiwix-build cache file

[kelson42]

• This is not really the right place even if these data are transitory
• We need a clear folder for that ci-cd_cache
• We should have a cleanup mechanism
• In addition the whole process/mgmt around this is unclear, see Add a section about cache mgmt in the README #561

[kelson42]

I would like to add, that tmp.kiwix.org is unavailable (only the ci directory is writable) to other usages (upload of temporary files by users)