cmake: Fix Windows issue running menuconfig in Command Prompt if winpty on Path

Previous version assumed having winpty on the Path meant that it would be needed,
but some tools (like Atlassian Sourcetree) permanently add winpty to the path.

Symptom is arrow keys not working in menuconfig dialog.

tools/cmake/kconfig.cmake

@@ -18,7 +18,11 @@ endmacro()
 
 if(CMAKE_HOST_WIN32)
     # Prefer a prebuilt mconf-idf on Windows
-    find_program(WINPTY winpty)
+    if(DEFINED ENV{MSYSTEM})
+        find_program(WINPTY winpty)
+    else()
+        unset(WINPTY CACHE)  # in case previous CMake run was in a tty and this one is not
+    endif()
     find_program(MCONF mconf-idf)
 
     # Fall back to the old binary which was called 'mconf' not 'mconf-idf'