home-manager: add coreutils to PATH

This ensures that we use the expected version of some useful tools,
such as readlink.

home-manager/default.nix

@@ -28,6 +28,7 @@ pkgs.stdenv.mkDerivation {
 
     substituteInPlace $out/bin/home-manager \
       --subst-var-by bash "${pkgs.bash}" \
+      --subst-var-by coreutils "${pkgs.coreutils}" \
       --subst-var-by MODULES_PATH '${modulesPath}' \
       --subst-var-by HOME_MANAGER_EXPR_PATH "${homeManagerExpr}"
   '';

home-manager/home-manager

@@ -1,5 +1,10 @@
 #!@bash@/bin/bash
 
+# This code explicitly requires GNU Core Utilities and we therefore
+# need to ensure they are prioritized over any other similarly named
+# tools on the system.
+PATH=@coreutils@/bin:$PATH
+
 set -euo pipefail
 
 function doBuild() {