lint_po: set XDG_CACHE_HOME independently (refs: #17359).

Let's clean things up a little: The previous commit was an incremental
evolution, creating individual cache directories below the previously
global temporary directory. Even if that works fine, that's a little
convoluted, and there's no reason for this indirection: each i18nspector
call can get its own temporary directory anywhere, so simplify how the
temporary directories are created.