[Build] Skip build jobs in the CreateInstaller stage during Image Validation pipeline runs

[alexlamtest]

In an Agg PR pipeline run that is validating:

• an arm test OS image, we can skip non-arm build jobs in the Createinstaller stage, as we won't be using those bits for testing an arm image.
• a non-arm test OS image, we can skip arm build jobs in the Createinstaller stage, as we won't be using those bits for testing a non-arm image.

This change on average reduces the work in the Createinstaller stage by a half in a test OS image validation pipeline run, saving time and resources.
Currently, we are yet not skipping a build job completely, but most steps in the job are no-op'ed.
A related PR against the internal Aggregator repo disables BinSkim in CreateInstaller jobs iff we are validating a test OS image, to avoid a warning.

I see that two x64 test jobs failed. These 2 test jobs failed the same way in other PRs w/o my changes.

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[alexlamtest]

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[alexlamtest]

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