[CMake]: Fix python command for mingw environment (#6984)

In mingw build environment, this fixes the build error:
'py' is not recognized as an internal or external command
The 'py' launcher is available for MSVC python only. More
https://docs.python.org/3/using/windows.html#from-the-command-line
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ca05a1e..2fc20b7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -523,7 +523,7 @@
   # Skip doing this if the MSVC version is below VS 12.
   # https://cmake.org/cmake/help/latest/variable/MSVC_VERSION.html
   (NOT MSVC OR MSVC_VERSION GREATER 1800))
-  if(WIN32)
+  if(MSVC)
     set(GENERATION_SCRIPT py scripts/generate_code.py) 
    else()
     set(GENERATION_SCRIPT scripts/generate_code.py)
@@ -750,4 +750,4 @@
 # Include for running Google Benchmarks.
 if(FLATBUFFERS_BUILD_BENCHMARKS AND CMAKE_VERSION VERSION_GREATER 3.13)
   add_subdirectory(benchmarks)
-endif()
\ No newline at end of file
+endif()