BUG: Use Python3_EXECUTABLE when specified

The FindPython3.cmake module will silently fail and set the
Python3_EXECUTABLE variable to Python3_EXECUTABLE-NOTFOUND even if it is
explicitly specified with
-DPython3_EXECUTABLE:FILEPATH=/the/path/we/know/we/want.

CMakeLists.txt

@@ -567,7 +567,12 @@ if(BUILD_TESTING OR ITK_BUILD_DOCUMENTATION OR ITK_WRAP_PYTHON)
     endif()
   else()
     if(ITK_WRAP_PYTHON)
+      if(DEFINED Python3_EXECUTABLE)
+        set(_specified_Python3_EXECUTABLE ${Python3_EXECUTABLE})
+      endif()
       find_package(Python3 COMPONENTS Interpreter Development)
+      set(Python3_EXECUTABLE ${_specified_Python3_EXECUTABLE} CACHE INTERNAL
+        "Path to the Python interpreter" FORCE)
     else()
       find_package(Python3 COMPONENTS Interpreter)
     endif()