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>

OvmfPkg/PlatformCI/.azurepipelines/Windows-VS2019.yml

@@ -132,7 +132,7 @@ jobs:
         build_flags: $(Build.Flags)
         run_flags: $(Run.Flags)
         extra_install_step:
-        - powershell: choco install qemu; Write-Host "##vso[task.prependpath]c:\Program Files\qemu"
+        - powershell: choco install qemu --version=2020.08.14; Write-Host "##vso[task.prependpath]c:\Program Files\qemu"
           displayName: Install QEMU and Set QEMU on path # friendly name displayed in the UI
           condition: and(gt(variables.pkg_count, 0), succeeded())