Exercise UseBuildManifest pipeline variable

[missymessa]

In the Post Build Signing feature, we introduced a new variable in our pipeline. We need to exercise the use of that variable, validating that all the expected functionality works.

Related PRs:

• Add build manifest #5210

Code that needs to be tested:

• https://github.com/dotnet/arcade/blob/master/azure-pipelines.yml
• https://github.com/dotnet/arcade/blob/master/eng/common-variables.yml
• https://github.com/dotnet/arcade/blob/master/eng/common/templates/job/job.yml
• https://github.com/dotnet/arcade/blob/master/eng/common/templates/post-build/post-build.yml

[missymessa]

Dupe of #6346