SystemTools: Fix FileIsExecutable on Windows

Since commit 66724af8 (SystemTools: Teach FindProgram to find
non-readable programs, 2020-04-04), `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.