Share more variable definitions

Author: AArnott

azure-pipelines/vs-validation.yml

@@ -32,10 +32,9 @@ extends:
     stages:
     - stage: Build
       variables:
-        DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
-        NUGET_PACKAGES: $(Agent.TempDirectory)/.nuget/packages/
-        BuildConfiguration: Release
-        SkipCodesignVerify: true
+      - template: /azure-pipelines/BuildStageVariables.yml@self
+      - name: SkipCodesignVerify
+        value: true
 
       jobs:
       - template: /azure-pipelines/build.yml@self