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>

doc/manual/src/command-ref/env-common.md

@@ -91,3 +91,16 @@ Most Nix commands interpret the following environment variables:
     variable sets the initial size of the heap in bytes. It defaults to
     384 MiB. Setting it to a low value reduces memory consumption, but
     will increase runtime due to the overhead of garbage collection.
+
+## XDG Base Directory
+
+New Nix commands conform to the [XDG Base Directory Specification], and use the following environment variables to determine locations of various state and configuration files:
+
+- [`XDG_CONFIG_HOME`]{#env-XDG_CONFIG_HOME} (default `~/.config`)
+- [`XDG_STATE_HOME`]{#env-XDG_STATE_HOME} (default `~/.local/state`)
+- [`XDG_CACHE_HOME`]{#env-XDG_CACHE_HOME} (default `~/.cache`)
+
+Classic Nix commands can also be made to follow this standard using the [`use-xdg-base-directories`] configuration option.
+
+[XDG Base Directory Specification]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
+[`use-xdg-base-directories`]: ../command-ref/conf-file.md#conf-use-xdg-base-directories