breakpointHook: add for debugging failing builds

Usuage: Add breakpointHook to your `buildInputs` like this:

  stdenv.mkDerivation rec {
    # ...
    buildInputs = [ breakpointHook ];
  });

When the build fails as show in this example:

  pkgs.hello.overrideAttrs (old: {
    buildInputs = [ breakpointHook ];
    postPatch = ''
      false
    '';
  });

It will halt execution printing the following message:

build failed in patchPhase with exit code 1
To attach to this build run the following command as root:

   cntr attach -t command cntr-/nix/store/ynyb4n82x2r7sldd58pbb405jdqh5f00-hello-2.10

Installing cntr and running the command will provide shell access to the
build sandbox of failed build:

sudo cntr attach -t command cntr-/nix/store/ynyb4n82x2r7sldd58pbb405jdqh5f00-hello-2.10
WARNING: bad ownership on /nix/var/nix/profiles/per-user/root, should be 1000
[nixbld@localhost:/var/lib/cntr]$

At /var/lib/cntr the sandbox filesystem is mounted. All commands and
files of the system are still accessible within the shell.
To execute commands from the sandbox use the `cntr exec` subcommand.