Fix workloads build by bringing back some needed variables

dotnet#99179 made some changes and switched the template the workloads build was using.
In the process, workloadArtifactsPath and workloadPackagesPath got dropped, which meant the workloads build did not
pick up any manifests to process. As a result, the build failed.

eng/pipelines/mono/templates/workloads-build.yml

@@ -42,6 +42,12 @@ jobs:
  value: '/p:OfficialBuildId=$(Build.BuildNumber)'
  - name: SignType
  value: $[ coalesce(variables.OfficialSignType, 'real') ]
+ - name: monoRepoRoot
+ value: '$(Build.SourcesDirectory)/src/mono'
+ - name: workloadPackagesPath
+ value: $(Build.SourcesDirectory)/artifacts/workloadPackages
+ - name: workloadArtifactsPath
+ value: $(Build.SourcesDirectory)/artifacts/workloads
  - ${{ parameters.variables }}
 
  steps: