Move the default profiles to the user’s home

Rather than using `/nix/var/nix/{profiles,gcroots}/per-user/`, put the user
profiles and gcroots under `$XDG_DATA_DIR/nix/{profiles,gcroots}`.

This means that the daemon no longer needs to manage these paths itself
(they are fully handled client-side). In particular, it doesn’t have to
`chown` them anymore (removing one need for root).

This does change the layout of the gc-roots created by nix-env, and is
likely to break some stuff, so I’m not sure how to properly handle that.

src/libstore/build/local-derivation-goal.cc

@@ -1517,7 +1517,7 @@ void LocalDerivationGoal::startDaemon()
                 try {
                     daemon::processConnection(store, from, to,
                         daemon::NotTrusted, daemon::Recursive,
-                        [&](Store & store) { store.createUser("nobody", 65535); });
+                        [&](Store & store) {});
                     debug("terminated daemon connection");
                 } catch (SysError &) {
                     ignoreException();