breakpointHook: prefer system-provided nsenter

pkgs/by-name/br/breakpointHook/package.nix

···
 
 let
   attach = buildPackages.writeShellScriptBin "attach" ''
-    export PATH="${
+    export PATH="''${PATH:+''${PATH}:}${
       lib.makeBinPath [
         buildPackages.bash
         buildPackages.coreutils