Linux cache placement should follow the XDG base specification

[sago007]

Currently capsules are unpacked to "$HOME/.capsule".

However according to the XDG Base specification it should be stored in "$XDG_CACHE_HOME/capsule" or fallback to "$HOME/.cache/capsule" if $XDG_CACHE_HOME is not defined (and it is almost never defined because .cache is such a sensible choice).

This is especially useful for cache files because you often want to exclude them from backups. It also matches the Windows behaviour of capsules. On Windows LOCALAPPDATA is chosen over APPDATA.