Flatpak builder cache should consider architecture

[phw]

Currently when calling flatpak-builder for the same JSON file with different architecture, this always invalidates the cache. Currently I work around this by creating separate build directories and copy the necessary files into them, but I think the easier solution would be for builder to consider the architecture and crate separate caches, e.g. by creating something like:

• .flatpak-builder/
  • build
    • i386
    • x86_64
  • cache
    • i386
    • x86_64