zisk/sp1: make guest crates standalone workspace roots

[samuelburnham]

The zkVM guest crates (zisk/guest, zisk/agg-guest, sp1/guest) are compiled out-of-tree for the riscv target by the zkVM build scripts, never as part of the host workspace. They carried no [workspace] of their own and relied purely on the ancestor manifests' exclude lists to be treated as standalone.

That breaks under a NESTED checkout. The benchmark base-run (bench-pr.yml) checks the base repo out under base/ INSIDE the PR checkout, so the base guest lives at base/zisk/guest. The root manifest's exclude = ["zisk", "sp1"] is relative, so it covers ./zisk but not ./base/zisk; cargo's workspace search for the base guest walks up past base/zisk (which excludes it) to the outer PR-checkout root, which neither lists nor excludes it, and ziskbuild's cargo metadata call panics with "current package believes it's in a workspace when it's not". The base-side zisk-host build then fails and the main column of the bench comparison comes up empty (seen on the zisk-InitStd-execute base run).

Give each guest its own empty [workspace] table so cargo pins its workspace to the crate itself and never walks up to an ancestor, regardless of nesting — exactly what the cargo error recommends. Verified by reproducing the nested layout: without the table cargo emits the exact error; with it, resolution no longer reaches the outer workspace. The guests use no workspace = true inheritance, so a standalone root changes nothing else.

Fixes the n/a errors for the base branch in the !benchmark output, e.g. #442 (comment) (workflow logs)