treewide: use pkgs.nixos-enter instead of config.system.build.nixos-enter

Author: K900

nixos/doc/manual/development/testing-installer.chapter.md

@@ -6,13 +6,13 @@ tedious, so here is a quick way to see if the installer works properly:
 ```ShellSession
 # mount -t tmpfs none /mnt
 # nixos-generate-config --root /mnt
-$ nix-build '<nixpkgs/nixos>' -A config.system.build.nixos-install
+$ nix-build '<nixpkgs>' -A nixos-install
 # ./result/bin/nixos-install
 ```
 
 To start a login shell in the new NixOS installation in `/mnt`:
 
 ```ShellSession
-$ nix-build '<nixpkgs/nixos>' -A config.system.build.nixos-enter
+$ nix-build '<nixpkgs>' -A nixos-enter
 # ./result/bin/nixos-enter
 ```

nixos/lib/make-disk-image.nix

@@ -321,7 +321,7 @@ let format' = format; in let
       e2fsprogs
       lkl
       config.system.build.nixos-install
-      config.system.build.nixos-enter
+      nixos-enter
       nix
       systemdMinimal
     ]

nixos/lib/make-multi-disk-zfs-image.nix

@@ -124,7 +124,7 @@ let
 
   tools = lib.makeBinPath (
     with pkgs; [
-      config.system.build.nixos-enter
+      nixos-enter
       config.system.build.nixos-install
       dosfstools
       e2fsprogs

nixos/lib/make-single-disk-zfs-image.nix

@@ -112,7 +112,7 @@ let
 
   tools = lib.makeBinPath (
     with pkgs; [
-      config.system.build.nixos-enter
+      nixos-enter
       config.system.build.nixos-install
       dosfstools
       e2fsprogs