EmulatorPkg: 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: Andrew Fish <afish@apple.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Nickle Wang <nickle.wang@hpe.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>

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

@@ -20,7 +20,7 @@ jobs:
   - job: Platform_CI
     variables:
       package: 'EmulatorPkg'
-      vm_image: 'windows-latest'
+      vm_image: 'windows-2019'
       should_run: true
       run_flags: "MAKE_STARTUP_NSH=TRUE"