home-environment: explicitly use coreutils

Before we used dirname and readlink from the ambient environment,
which caused problems when they don't behave as expected.

Fixes #3516

modules/home-environment.nix

@@ -679,7 +679,6 @@ in
             gnused
             ncurses             # For `tput`.
           ]
-          ++ optional (config.nix.enable && config.nix.package != null) config.nix.package
           ++ config.home.extraActivationPath
         )
         + (
@@ -688,7 +687,7 @@ in
           if config.nix.enable && config.nix.package != null then
             ":${config.nix.package}/bin"
           else
-            ":$(dirname $(readlink -m $(type -p nix-env)))"
+            ":$(${pkgs.coreutils}/bin/dirname $(${pkgs.coreutils}/bin/readlink -m $(type -p nix-env)))"
         )
         + optionalString (!cfg.emptyActivationPath) "\${PATH:+:}$PATH";