Enable BinSkim scan in nightly validation

[MilenaHristova]

Summary of the changes

Enabling BinSkim scan over build artifacts in Validate-DotNet pipeline and the .NET staging pipeline based on company requirements.

We are required to run SDL tools on official builds and implement automated bug filling for the tools output. Currently we are running SDL checks over the source code in the nightly builds and in the .NET staging pipeline, but to be compliant we need to also run BinSkim over the produced artifacts.

This PRs is enabling BinSkim checks in the nightly run of Validate-DotNet pipeline.

More information is in the Automate BinSkim runs over official builds issue

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• I've posted a comment on an issue with a detailed description of how I am planning to contribute and got approval from a member of the team
• The code builds and tests pass locally (also verified by our automated build checks)
• Commit messages follow this format:

        Summary of the changes
        - Detail 1
        - Detail 2

        Fixes #bugnumber

• Tests for the changes have been added (for bug fixes / features)
• Code follows the same patterns and style as existing code in this repo