OvmfPkg: Use windows-2019 VM image

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3890

The YAML file previously specified "windows-latest" which recently
moved to VS2022. To continue using VS2019 (at least in the short
term), the VM image needs to be explicitly set to "windows-2019".

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

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

@@ -18,7 +18,7 @@ jobs:
   - job: Platform_CI
     variables:
       package: 'OvmfPkg'
-      vm_image: 'windows-latest'
+      vm_image: 'windows-2019'
       should_run: true
       run_flags: "MAKE_STARTUP_NSH=TRUE QEMU_HEADLESS=TRUE"