[CMake][Exp PyROOT] Force Python related CMake variables into CMakeCache

After investigating the bug reported here

https://sft.its.cern.ch/jira/browse/ROOT-10626

we noticed that the deprecated variables
PYTHON_EXECUTABLE
PYTHON_INCLUDE_DIRS
PYTHON_LIBRARIES
PYTHON_VERSION_MAJOR
PYTHON_VERSION_MINOR
PYTHON_VERSION_STRING
NUMPY_FOUND
NUMPY_INCLUDE_DIRS
are not set correctly in the CMakeCache. This should fix the problem.

cmake/modules/SearchRootCoreDeps.cmake

@@ -49,14 +49,14 @@ if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.14)
     # Search for NumPy, but not required:
     find_package(Python2 COMPONENTS NumPy QUIET)
     # Compat with find_package(PythonInterp), find_package(PythonLibs)
-    set(PYTHON_EXECUTABLE "${Python2_EXECUTABLE}")
-    set(PYTHON_INCLUDE_DIRS "${Python2_INCLUDE_DIRS}")
-    set(PYTHON_LIBRARIES "${Python2_LIBRARIES}")
-    set(PYTHON_VERSION_STRING "${Python2_VERSION}")
-    set(PYTHON_VERSION_MAJOR "${Python2_VERSION_MAJOR}")
-    set(PYTHON_VERSION_MINOR "${Python2_VERSION_MINOR}")
-    set(NUMPY_FOUND ${Python2_NumPy_FOUND})
-    set(NUMPY_INCLUDE_DIRS "${Python2_NumPy_INCLUDE_DIRS}")
+    set(PYTHON_EXECUTABLE "${Python2_EXECUTABLE}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_INCLUDE_DIRS "${Python2_INCLUDE_DIRS}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_LIBRARIES "${Python2_LIBRARIES}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_VERSION_STRING "${Python2_VERSION}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_VERSION_MAJOR "${Python2_VERSION_MAJOR}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_VERSION_MINOR "${Python2_VERSION_MINOR}" CACHE INTERNAL "" FORCE)
+    set(NUMPY_FOUND ${Python2_NumPy_FOUND} CACHE INTERNAL "" FORCE)
+    set(NUMPY_INCLUDE_DIRS "${Python2_NumPy_INCLUDE_DIRS}" CACHE INTERNAL "" FORCE)
   else()
     if(PYTHON_EXECUTABLE AND NOT Python_EXECUTABLE)
       set(Python_EXECUTABLE "${PYTHON_EXECUTABLE}")
@@ -71,14 +71,14 @@ if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.14)
     # Search for NumPy, but not required:
     find_package(Python COMPONENTS NumPy QUIET)
     # Compat with find_package(PythonInterp), find_package(PythonLibs), find_package(NumPy)
-    set(PYTHON_EXECUTABLE "${Python_EXECUTABLE}")
-    set(PYTHON_INCLUDE_DIRS "${Python_INCLUDE_DIRS}")
-    set(PYTHON_LIBRARIES "${Python_LIBRARIES}")
-    set(PYTHON_VERSION_STRING "${Python_VERSION}")
-    set(PYTHON_VERSION_MAJOR "${Python_VERSION_MAJOR}")
-    set(PYTHON_VERSION_MINOR "${Python_VERSION_MINOR}")
-    set(NUMPY_FOUND ${Python_NumPy_FOUND})
-    set(NUMPY_INCLUDE_DIRS "${Python_NumPy_INCLUDE_DIRS}")
+    set(PYTHON_EXECUTABLE "${Python_EXECUTABLE}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_INCLUDE_DIRS "${Python_INCLUDE_DIRS}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_LIBRARIES "${Python_LIBRARIES}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_VERSION_STRING "${Python_VERSION}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_VERSION_MAJOR "${Python_VERSION_MAJOR}" CACHE INTERNAL "" FORCE)
+    set(PYTHON_VERSION_MINOR "${Python_VERSION_MINOR}" CACHE INTERNAL "" FORCE)
+    set(NUMPY_FOUND ${Python_NumPy_FOUND} CACHE INTERNAL "" FORCE)
+    set(NUMPY_INCLUDE_DIRS "${Python_NumPy_INCLUDE_DIRS}" CACHE INTERNAL "" FORCE)
   endif()
 
 else()