Fix Robust.Benchmarks failing to compile

[ArtisticRoomba]

Sometime around 787330f5c6c256c53c2ed6f9b0909ab4265476a5, Robust.Benchmarks (and only Robust.Benchmarks, Client.Benchmarks wasn't throwing a fit) was giving me a hard time over this error and similar missing references:

/home/roomba/RiderProjects/space-station-14/RobustToolbox/bin/Benchmarks/Robust.Benchmarks-Job-JMDAGQ-1.notcs(712,48):
error CS0234: The type or namespace name 'Engines' does not exist in the namespace 'BenchmarkDotNet' (are you missing an assembly reference?)
[/home/roomba/RiderProjects/space-station-14/RobustToolbox/bin/Benchmarks/Robust.Benchmarks-Job-JMDAGQ-1/BenchmarkDotNet.Autogenerated.csproj]

By default the RT project privates package references so the autogenerated files could not reference a built BenchmarkDotNet.dll through Robust.Benchmarks when BenchmarkDotNet needed to build its autogenerated runner.

Client.Benchmarks works because we don't suppress package references up there.

The fix is to just make it not private.