RISC-V: Reinstate Hermit installer script

What

Reinstate the Hermit installer script (removed in !17831 (merged)), which is still used in the SDK. Make changes to src/riscv trigger the test_kernels job.

Why

The test_kernels job currently fails due to the missing Hermit installer script.

Manually Testing

make -f kernels.mk check
make -f kernels.mk test

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.