RISC-V: Simplify workspace and asset update process

What

• Simplifies the workspace in src/riscv since it now only contains the api package.
• Changes the process for updating the dependency on riscv-pvm and the checked-in assets:
  • All updated via a single make rule (update-riscv-pvm).
  • Binaries are downloaded directly from the repo's assets instead of installing them from main
  • The initial jstz proof asset is also updated.

Why

To simplify the workflow of keeping the monorepo up to date with riscv-pvm.

The compilation of the dummy kernel and jstz binaries has been replaced with a download of the checked-in assets in order to sync the jstz binary with its initial proof regression file. Since the assets checked in to riscv-pvm are themselves manually updated, they diverge from the latest main (which is what the installation was based on).

The workspace in src/riscv could in principle be removed completely but this requires some broader changes to CI which I am leaving as future work.

Manually Testing

To run the update rule:

update-riscv-pvm

Benchmarking

Not applicable.

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
  • make -C src/riscv
  • dune build src/lib_riscv
  • dune build src/rust_deps
• Benchmark performance and populate the table above if needed.
• Explain changes to regression test captures when applicable.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.