To quote the XDG specification:
There is a single base directory relative to which user-specific
data files should be written. This directory is defined by the\
environment variable $XDG_DATA_HOME.
Rather than adding another directory to $HOME, I think that it's better
to follow this standard to avoid a cluttered home-dir.
Fixes the broken metrics evaluation which was caused by a `trace`
warning in stdout which confused `jq` in `pkgs/top-level/metrics.nix`.
Also made the `bind-device` feature optional as suggested after the
merge.
