Disable pre-build validation stage in this repo

[lbussell]

There are no pre-build validation steps in this repo (docker-tools). Unless they are added, this step should be removed from our pipeline as it only wastes time. It also causes a warning when we try to upload test results when there are no tests to run.

https://github.com/dotnet/docker-tools/blob/3d4a494c5e7a9a741b7359f220ef70a779ce1dc7/src/Microsoft.DotNet.ImageBuilder/run-tests.ps1#L29-L31

[dotnet-issue-labeler[bot]]

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[lbussell]

[Triage] It's important to keep this PreBuildValidation running in the eng-validation pipeline in order to exercise that pipeline code. We could synthesize some fake or empty trx file to test the publishing step.

We should also figure out why this warning isn't happening in the eng-validation internal pipeline while it is happening in the imagebuilder official pipeline.