This stores the cache in the canonical format (i.e. uid/gid 0 and no
weird permissions). This has two advantages, first of all it matches
what flatpak build-export will produce, so diff:ing with the final
result will make things easier to read, shared repos will be smaller,
etc. Secondly, it will allow us to switch to bare-user-only mode which
means we don't need/use xattrs for the build filesystem.
Note: We bump the cache format as the cache will change affecting
e.g. ostree diff between different cachepoints, so this will rebuild
everything once.
Closes: #80
Approved by: alexlarsson