Set the original build number via logging command

Author: sharwell

azure-pipelines.yml

@@ -11,7 +11,6 @@ jobs:
   variables:
     BuildSolution: StyleCopAnalyzers.sln
     BuildPlatform: Any CPU
-    OriginalBuildNumber: $(Build.BuildNumber)
   strategy:
     matrix:
       Debug:
@@ -21,6 +20,9 @@ jobs:
         BuildConfiguration: Release
         _debugArg: ''
   steps:
+  - powershell: Write-Host "##vso[task.setvariable variable=OriginalBuildNumber;]$(Build.BuildNumber)"
+    displayName: Set original build number
+
   - task: NuGetToolInstaller@0
     displayName: 'Use NuGet 5.3.1'
     inputs: