OvmfPkg/PlatformCI/.azurepiplines: Use stable release of QEMU

OvmfPkg/PlatformCI/.azurepiplines: Use stable release of QEMU

Install the 2020.08.14 release of QEMU for Windows.

The QEMU release from 2020.11.20 is installed into the incorrect
directory and is causing EDK II CI failures in the run to shell
step.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>