Eliminate some flakes

- Get ASAN working on Windows.

- Deleting directories and then recreating them with the same name in a
  short period of time appears to be a no-no on Windows.

- There's no reason to call FlushFileBuffers on close() for pipes, and
  it's harmful since it might block indefinitely for no good reason.

libc/rand/rand64.c

@@ -33,7 +33,7 @@ hidden extern uint64_t g_rando64;
  *
  * @see rngset()
  */
-nodebuginfo uint64_t(rand64)(void) {
+nodebuginfo uint64_t rand64(void) {
   uint64_t res;
   if (X86_HAVE(RDRND)) {
     res = rdrand();