Revert "Remove repo level SB legs in CI and PR validation (#45765)"

[mthalman]

This reverts commit cc260aa.

By not having a SB leg, it causes the sdk CI to no longer publish SB intermediates. This is a problem for repos that consume sdk's SB intermediates: dotnet/runtime#111328 (comment).

[mmitche]

Are you sure that reintroducing the repo level leg is the correct thing to do here? I think it might be to just baseline those packages in runtime.

[mthalman]

But then there'd be no way to get updated intermediates flowed going forward.

[MichaelSimons]

I thought the same thing as @mmitche. What would it look like to remove the runtime's sdk intermediate dependency and just add the prebuilts to the baseline. This is in context of the decision that we will be removing the repo SB legs in the near future. This decision should be which ever is less work. Keeping the sdk repo's SB leg working as been a non-zero cost which was the motivation for removing it.

[mmitche]

But then there'd be no way to get updated intermediates flowed going forward.

Do they need them? I don't think there is useful information that comes out of having that set of intermediates flow into runtime that isn't already produced by the stage2 build of the VMR.

[mthalman]

Closing this in favor of allowing the SDK-related prebuilts in dotnet/runtime#111328 instead of relying on the SB intermediate.