os: terminate windows processes via handle directly

We already have a handle to the process, so use that for termination,
rather than doing a new lookup based on the PID.

Change-Id: I2958c1817f12f3dd783412baacbf629049f6956a
Reviewed-on: https://go-review.googlesource.com/c/go/+/322509


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>