Add --exclude to buck2 build

[cbarrete]

buck2 test let's the user exclude some tests based on the labels that are attached to them. Would you accept a PR that adds the same feature to buck2 build? This would be very convenient to improve local iteration times when building all targets in projects that have their build times dominated by infrequently useful targets (e.g. release artifacts, container images, etc.).

My understanding is that labels are a feature that will eventually be replaced, but this would deliver significant value at a low cost in the meantime.