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>

scripts/install-nix-from-closure.sh

@@ -188,6 +188,8 @@ fi
 # shellcheck source=./nix-profile.sh.in
 . "$nix/etc/profile.d/nix.sh"
 
+NIX_LINK="$HOME/.nix-profile"
+
 if ! "$nix/bin/nix-env" -i "$nix"; then
     echo "$0: unable to install Nix into your default profile" >&2
     exit 1
@@ -196,7 +198,7 @@ fi
 # Install an SSL certificate bundle.
 if [ -z "$NIX_SSL_CERT_FILE" ] || ! [ -f "$NIX_SSL_CERT_FILE" ]; then
     "$nix/bin/nix-env" -i "$cacert"
-    export NIX_SSL_CERT_FILE="$HOME/.nix-profile/etc/ssl/certs/ca-bundle.crt"
+    export NIX_SSL_CERT_FILE="$NIX_LINK/etc/ssl/certs/ca-bundle.crt"
 fi
 
 # Subscribe the user to the Nixpkgs channel and fetch it.
@@ -214,8 +216,8 @@ fi
 
 added=
 p=
-p_sh=$HOME/.nix-profile/etc/profile.d/nix.sh
-p_fish=$HOME/.nix-profile/etc/profile.d/nix.fish
+p_sh=$NIX_LINK/etc/profile.d/nix.sh
+p_fish=$NIX_LINK/etc/profile.d/nix.fish
 if [ -z "$NIX_INSTALLER_NO_MODIFY_PROFILE" ]; then
     # Make the shell source nix.sh during login.
     for i in .bash_profile .bash_login .profile; do