Summary output at startup makes no sence at `bst track` time

When launching a buildstream task which uses the scheduler, a pipeline summary is printed to the console with the logging header.

This makes sense at bst fetch and bst build time, but for bst track the element state and cache keys are nonsensical, since the result of bst track will cause previously cached elements to not be cached and will cause cache keys to change.

Either we should disable this summary at bst track time, or we should set some internal state so that the output makes more sense.