fixing variable name in build

Author: eiximenis

build/azure-devops/buildimages.yaml

@@ -52,7 +52,7 @@ jobs:
         TAG=${{ variables['Build.SourceBranchName'] }} 
   - task: CopyFiles@2
     inputs:
-      sourceFolder: ${{ variables.Build.SourcesDirectory }}/k8s/helm    
+      sourceFolder: ${{ variables['Build.SourcesDirectory'] }}/k8s/helm    
       targetFolder: ${{ variables['Build.ArtifactStagingDirectory'] }}/k8s/helm
   - task: PublishBuildArtifacts@1
     inputs: