With https://github.com/google/nixery/pull/127, nixery will use extended
attributes to store metadata (when using local storage).
Right now, our integration test mounts a tmpfs to /var/cache/nixery.
However, *user* xattrs aren't supported with tmpfs [1], so setting
xattrs would fail.
To workaround this, use a folder in the current working directory and
hope it's backed by something supporting user xattrs (which is the case
for GitHub Actions).
[1]: https://man7.org/linux/man-pages/man5/tmpfs.5.html#NOTES
