build: make sure the AnyCPU build sets BuildPlatform properly (#19925)

This caused the WPF-only build (packaging phase, really) to fail.

Fixes d6714f3 (#19328)

(cherry picked from commit 0f6ee4a)
Service-Card-Id: PVTI_lADOAF3p4s4BQX0-zgmftD0
Service-Version: 1.25

Author: DHowett

build/pipelines/templates-v2/job-build-package-wpf.yml

@@ -46,6 +46,7 @@ jobs:
           BuildConfiguration: ${{ config }}
   dependsOn: ${{ parameters.dependsOn }}
   variables:
+    BuildPlatform: Any CPU
     OutputBuildPlatform: AnyCPU
     Terminal.BinDir: $(Build.SourcesDirectory)/bin/$(OutputBuildPlatform)/$(BuildConfiguration)
     JobOutputDirectory: $(Build.ArtifactStagingDirectory)\nupkg