.azurepipelines: Use windows-2019 VM image

.azurepipelines: 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: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
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>