A setting to follow XDG Base Directory standard

XDG Base Directory is a standard for locations for storing various
files. Nix has a few files which seem to fit in the standard, but
currently use a custom location directly in the user's ~, polluting
it:

- ~/.nix-profile
- ~/.nix-defexpr
- ~/.nix-channels

This commit adds a config option (use-xdg-base-directories) to follow
the XDG spec and instead use the following locations:

- $XDG_STATE_HOME/nix/profile
- $XDG_STATE_HOME/nix/defexpr
- $XDG_STATE_HOME/nix/channels

If $XDG_STATE_HOME is not set, it is assumed to be ~/.local/state.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>
Co-authored-by: Tim Fenney <kodekata@gmail.com>
Co-authored-by: pasqui23 <pasqui23@users.noreply.github.com>
Co-authored-by: Artturin <Artturin@artturin.com>
Co-authored-by: John Ericson <Ericson2314@Yahoo.com>

tests/nix-channel.sh

@@ -12,6 +12,19 @@ nix-channel --remove xyzzy
 [ -e $TEST_HOME/.nix-channels ]
 [ "$(cat $TEST_HOME/.nix-channels)" = '' ]
 
+# Test the XDG Base Directories support
+
+export NIX_CONFIG="use-xdg-base-directories = true"
+
+nix-channel --add http://foo/bar xyzzy
+nix-channel --list | grep -q http://foo/bar
+nix-channel --remove xyzzy
+
+unset NIX_CONFIG
+
+[ -e $TEST_HOME/.local/state/nix/channels ]
+[ "$(cat $TEST_HOME/.local/state/nix/channels)" = '' ]
+
 # Create a channel.
 rm -rf $TEST_ROOT/foo
 mkdir -p $TEST_ROOT/foo