feat(l2): support Pico backend

[xqft]

Support was added to ethrex in lambdaclass/ethrex#5440

[Copilot]

Pull request overview

This PR adds support for the Pico backend as a new ZK-VM option for the L2 system, following the integration work done in the ethrex repository (PR #5440).

Key changes:

• Adds Pico as a supported backend alongside SP1, Risc0, ZisK, and OpenVM
• Updates all ethrex dependencies to use the "pico" branch instead of "main"
• Introduces new Pico-specific dependencies and Plonky3 libraries from brevis-network fork
• Adds CI/CD workflow support for building and testing with the Pico feature

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/cli.rs	Adds Pico backend case to the ZKVM match statement and updates the error message to include Pico
Cargo.toml	Changes all ethrex dependency branches from "main" to "pico" and adds the pico feature flag
Cargo.lock	Updates dependency lock file with new Pico-related packages (pico-sdk, pico-vm, pico-derive, etc.) and Plonky3 libraries from brevis-network fork
.github/workflows/pr-main.yaml	Adds "pico" and "l2,pico" to the feature matrix and adds conditional logic to use nightly Rust for Pico builds
.github/actions/install-pico/action.yml	Defines a new composite action to install the Pico toolchain and CLI

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