main: default to XDG cache directory for non-root users

[guillermodotn]

Summary

• Default the --cache path to $XDG_CACHE_HOME/image-builder/store (or ~/.cache/image-builder/store) when running as a non-root user, instead of the root-only /var/cache/image-builder/store
• Add an early writability check on the cache directory to fail fast with an actionable hint, instead of waiting for osbuild to fail after manifest generation

Fixes #76

[supakeen]

Thank you; this is pretty much what I had in mind for this.

Note that we do ship a systemd tmpfiles dropin to clean up /var/cache/image-builder but we don't do this for the new locations. You don't need to address this but I did want to mention it in this PR.

In the future we should grow a image-builder cache ... subcommand to manage/prune the cache.

[brlane-rht]

I'd like to see tests that always run since right now they are going to miss coverage depending on how they are invoked. The simple solution is to pass in os.Getuid to DefaultCacheDir, but that's kinda ugly.

[guillermodotn]

I'd like to see tests that always run since right now they are going to miss coverage depending on how they are invoked. The simple solution is to pass in os.Getuid to DefaultCacheDir, but that's kinda ugly.

Changed the function to take uid as a parameter, so now tests always run.