ci: Matrix job to check downstream compiles

[samuelburnham]

After #87 (review)

Each crate has its own dependencies and configuration, which could be handled by a repo-specific setup.yml action. However, in the case of Sphinx and ZKLC, these setup actions are largely redundant, but that can be streamlined by e.g. ZKLC just using the Sphinx setup action since it clones the latter anyway.

Another approach would be passing multiple crates to the check-downstream-compiles action, though it becomes a bit harder to debug if collapsed into a single step.