Proto/RISC-V: Protocol to use RISCV PVM from protocol environment

Closes of RV-374

What

Based on !16554 (merged) & !15918 (merged)

Use the riscv pvm from the protocol environment instead of the current dummy implementation.

Note: This MR is blocked on V15 beign activated in proto_alpha. If V14 were to be used, it would break linking for past protocols which are not using the environment module for RISCV anyway. V15 has been activated.

Why

This change ties the protocol implementation of the riscv pvm to the actual implementation in the rust library.

This change makes the RISCV pvm protocol implementation in src/proto_alpha/lib_protocol and the environment in src/lib_protocol_environment to not have stubs. All remaining stubs will be dealt with in the riscv modules from src/lib_riscv.

How

Deleted the stubbed module Riscv_proto_env and instead used the actual implementation made available in the protocol environment in !15918 (merged)

Manually testing the MR

# riscv specific tezt tests:
make -C src/riscv build
make
dune exec dune exec tezt/tests/main.exe -- riscv

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR