Add creating portable cache directory (`XDG_CACHE_HOME`)

[fiftydinar]

Introduced in uruntime since v0.4.4 release:
https://github.com/VHSgunzo/uruntime/releases/tag/v0.4.4

For other AppImages on older or different runtime, it would just be an unused directory, so it wouldn't harm anything.

Option would be included in --portable & new --portable-cache.

[QaidVoid]

Done in 09787c2