nixos: add nix package to activation script path

Fixes #2178

modules/home-environment.nix

@@ -373,6 +373,16 @@ in
       description = "The package containing the complete activation script.";
     };
 
+    home.extraActivationPath = mkOption {
+      internal = true;
+      type = types.listOf types.package;
+      default = [ ];
+      description = ''
+        Extra packages to add to <envar>PATH</envar> within the activation
+        script.
+      '';
+    };
+
     home.extraBuilderCommands = mkOption {
       type = types.lines;
       default = "";
@@ -570,7 +580,8 @@ in
 
         # Programs that always should be available on the activation
         # script's PATH.
-        activationBinPaths = lib.makeBinPath [
+        activationBinPaths = lib.makeBinPath (
+          [
             pkgs.bash
             pkgs.coreutils
             pkgs.diffutils        # For `cmp` and `diff`.
@@ -578,7 +589,8 @@ in
             pkgs.gnugrep
             pkgs.gnused
             pkgs.ncurses          # For `tput`.
-        ]
+          ] ++ config.home.extraActivationPath
+        )
         + optionalString (!cfg.emptyActivationPath) "\${PATH:+:}$PATH";
 
         activationScript = pkgs.writeShellScript "activation-script" ''

nixos/default.nix

@@ -32,6 +32,10 @@ let
 
           home.username = config.users.users.${name}.name;
           home.homeDirectory = config.users.users.${name}.home;
+
+          # Make activation script use same version of Nix as system as a whole.
+          # This avoids problems with Nix not being in PATH.
+          home.extraActivationPath = [ config.nix.package ];
         };
       })
     ] ++ cfg.sharedModules;