Temporarily block out UnmanagedCallersOnlyTest in Pri1 runs

[trylek]

Works around: #42168

/cc: @dotnet/runtime-infrastructure

[trylek]

The intent of this PR is to provide a temporary workaround before #42146 is merged in. I don't have the latest info on the development on 42146 so I'm just pulling in the proximate stakeholders to cooperate in making a call on which change should go in first. The idea is that, if 42146 is about to be merged in [very] soon, this change doesn't make sense and would only cause extra churn; but prolonged wait in the broken state is detrimental to all parties.

[jkotas]

cc @AaronRobinsonMSFT

[jkotas]

I don't have the latest info on the development on #42146

The test is failing with the current PR state, so it needs some work before it gets merged.

[ghost]

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[BruceForstall]

Interesting way to disable the build. Isn't the "normal" way to use <DisableProjectBuild>true</DisableProjectBuild>?

Anyway, LGTM if the outerloop job works

[AaronRobinsonMSFT]

I just merged the fixed test. This is no longer needed.