nixos/tests/acme: Use overridePythonAttrs

Quoting from @FRidh:

  Note overridePythonAttrs exists since 17.09. It overrides the call to
  buildPythonPackage.

While it's not strictly necessary to do this, because postPatch ends up
in drvAttrs anyway, it's probably better to use overridePythonAttrs so
we don't run into problems when the underlying implementation of
buildPythonPackage changes.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>