SystemTools: Fix FileIsExecutable on Windows

Since !175 (merged), FindProgram uses the new FileIsExecutable method. On Windows this changed the check from GetFileAttributesW to _waccess, but the latter returns success even for Windows Store stubs like

%USERPROFILE%\AppData\Local\Microsoft\WindowsApps\python.exe

Since the goal of the original change was to fix executable permissions checks on UNIX-like platforms, simply restore the old check code on Windows.