runners: stm32cubeprogrammer: fix program files path for win x64

The '%ProgramFiles%' is not guaranteed to be 'C:\Program Files' on
Windows x64.

The value will be 'C:\Program Files (x86)' if the west is executed by
32-bit Python.

To correct the issue '%ProgramW6432%' must be used.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>