Python detection update

Author: Ovascos

test/python/CMakeLists.txt

@@ -1,7 +1,7 @@
 if(LIBPOLY_BUILD_PYTHON_API)
     add_custom_target(check
         COMMAND
-            PYTHONPATH=${CMAKE_BINARY_DIR}/python ${Python3_INTERPRETER} ${CMAKE_CURRENT_SOURCE_DIR}/check.py
+            PYTHONPATH=$ENV{PYTHONPATH}:${CMAKE_BINARY_DIR}/python ${Python3_INTERPRETER} ${CMAKE_CURRENT_SOURCE_DIR}/check.py
         DEPENDS
             polypy
         WORKING_DIRECTORY