cmd/go: prefix calls to Windows rmdir with cmd.exe

rmdir is a built-in of cmd.exe. It's also an alias in powershell.exe. We
want always the cmd.exe on, so specify it explicitly.

Fixes #35813

Change-Id: I89723e993ee26a20b42d03b8a725ff10ccf30505
Reviewed-on: https://go-review.googlesource.com/c/go/+/208639


Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>