set full path without which

using which only works, if `generate_operators` is in `PATH`, otherwise cmake fails. This is especially the case if dune-codegen is configured completely new.

cmake/modules/DuneCodegenMacros.cmake

@@ -174,11 +174,8 @@ function(add_generated_executable)
 
   if(DUNE_CODEGEN_PROFILING)
     # This is a bit silly, but cProfile only finds entry point scripts
-    # if their full path is provided. So we resort to using which.
-    dune_execute_process(COMMAND which generate_operators
-                         OUTPUT_VARIABLE fullcommand
-                         OUTPUT_STRIP_TRAILING_WHITESPACE
-                         )
+    # if their full path is provided. 
+    set(fullcommand "${DUNE_PYTHON_VIRTUALENV_PATH}/bin/generate_operators")
   endif()
 	
   # Define build rules for all operator header files and gather a list of them