Ensure SignCheckTask in SigningValidation is being exercised in Arcade Validation

[missymessa]

SigningValidation is enabled in Arcade Validation, we should be sure that the SignCheckTask within it is being exercised.

Related PRs:

• Add build manifest #5210

Code that needs testing:

• https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Arcade.Sdk/tools/SdkTasks/SigningValidation.proj