env/windows-arm64/aws: halt after VM quits

The Windows ARM64 VM should exit upon build completion. Afterwards, we
should try to terminate our instance. The coordinator will clean it up
soon anyway.

For golang/go#42604

Change-Id: Ib7b6adbe293d8ea9e54c8288de9f10c6a45ebeb3
Reviewed-on: https://go-review.googlesource.com/c/build/+/322657
Trust: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

env/windows-arm64/aws/win10-arm64.sh

@@ -26,3 +26,5 @@
   -chardev file,path=/var/log/qemu-serial.log,id=char0 \
   -serial chardev:char0 \
   -device ramfb
+
+/sbin/halt