NIX_PATH: don't prepend $HOME-based value in session variable, set later

environment.sessionVariables cannot refer to the values of env vars,
and as a result this has caused problems in a variety of scenarios.

One use for these is that they're injected into /etc/profile,
elewhere these are used to populate an 'envfile' for pam
(`pam 5 pam_env.conf`) which mentions use of HOME being
potentially problematic.

Anyway if the goal is to make things easier for users,
simply do the NIX_PATH modification as extraInit.

This fixes the annoying problems generated by the current approach
(#40165 and others) while hopefully serving the original goal.

One way to check if things are borked is to try:

$ sudo env | grep NIX_PATH

Which (before this change) prints NIX_PATH variable with
an unexpanded $HOME in the value.

-------

This does mean the following won't contain user channels for 'will':
$ sudo -u will nix-instantiate --eval -E builtins.nixPath

However AFAICT currently they won't be present either,
due to unescaped $HOME.  Unsure if similar situation for other users
of sessionVariables (not sudo) work with current situation
(if they exist they will regress after this change AFAIK).

(cherry picked from commit fa67ca1)

nixos/modules/services/misc/nix-daemon.nix

@@ -345,7 +345,6 @@ in
         type = types.listOf types.str;
         default =
           [
-            "$HOME/.nix-defexpr/channels"
             "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
             "nixos=/nix/var/nix/profiles/per-user/root/channels/nixos"
             "nixos-config=/etc/nixos/configuration.nix"
@@ -437,7 +436,7 @@ in
 
     # Set up the environment variables for running Nix.
     environment.sessionVariables = cfg.envVars //
-      { NIX_PATH = concatStringsSep ":" cfg.nixPath;
+      { NIX_PATH = cfg.nixPath;
       };
 
     environment.extraInit = optionalString (!isNix20)
@@ -447,6 +446,8 @@ in
         if [ "$USER" != root -o ! -w /nix/var/nix/db ]; then
             export NIX_REMOTE=daemon
         fi
+      '' + ''
+        export NIX_PATH="$HOME/.nix-defexpr/channels''${NIX_PATH:+:$NIX_PATH}"
       '';
 
     nix.nrBuildUsers = mkDefault (lib.max 32 cfg.maxJobs);