First, "XDG_CONFIG_HOME" shouldn't be named "home", as it may be
confusing compared with `$HOME`, which an upcoming test will be using.
Then, using a fixed location for the test is problematic. Use
`$TEST_ROOT` instead.
Motivation: maintain project-level configuration files.
Document the whole situation a bit better so that it corresponds to the
implementation, and add NIX_USER_CONF_FILES that allows overriding
which user files Nix will load during startup.