Fix dune-gdt-super

The CI builds in the dune-gdt-super module are currently failing due to a failure in the dune-pybindxi cmake part

-- using C macro definitions from /home/dune-ci/src/dune-common/doc/doxygen/doxygen-macros for Doxygen
CMake Error at /home/dune-ci/src/dune-xt/cmake/modules/DunePybindxiInstallPythonPackage.cmake:37 (message):
  dune_python_install_package: Requested installations, but pip was not
  found!
Call Stack (most recent call first):
  CMakeLists.txt:66 (dune_pybindxi_install_python_package)
-- Configuring incomplete, errors occurred!

I am not sure whether this is a problem with the CI environment or a bug in DunePybindxiInstallPythonPackage.cmake. I am not getting this error locally.