ci(template-e2e): install Risc Zero toolchain for the rendered guest build

[danisharora099]

Problem

The Template E2E workflow's render-and-build matrix (default + lez-framework) has been failing on every run, across unrelated PRs and dates — e.g. #221's first run and #210 (wozos/scaffold-deslop) going back to early June. It is not a required check, so it hasn't blocked merges, but it leaves a persistent red ✗ on any PR that touches src/constants.rs, templates/**, or src/template/**.

Root cause

The job's final step renders a project, builds real lez/spel via setup, asserts doctor is clean, then runs lgs build on the rendered workspace. That last step cross-compiles the template's risc0 guest crate (methods/) to riscv32im-risc0-zkvm-elf, which needs the Risc Zero Rust toolchain. The workflow never installed it, so the guest build script panics:

thread 'main' panicked at risc0-build-3.0.5/src/lib.rs:379:
Risc Zero Rust toolchain not found. Try running `rzup install rust`

Everything earlier in the job (render → setup's full LEZ/spel build → circuits fetch → doctor zero-FAIL) already passes; this is the only failing step.

Fix

Add an rzup step before the build to install and register the risc0 rustup toolchain that risc0-build discovers. One added step, no other behavior change.

Notes

• Split out as its own PR (not folded into the circuits/basecamp Phase 2 PRs) since it's shared CI infra unrelated to those changes.
• Once merged, the Phase 2 branches pick it up on rebase and their render-and-build jobs go green.

[Copilot]

Pull request overview

Fixes the Template E2E workflow’s render-and-build job by installing the Risc Zero Rust toolchain needed to compile the rendered template’s risc0 guest crate during the final workspace build.

Changes:

• Add an rzup installation step to install/register the risc0 rustup toolchain before building the rendered project.
• Document why the toolchain is required (risc0 guest cross-compilation).

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[danisharora099]

Addressed both review nits in 4a6fe7c: switched to curl -fsSL so an HTTP error page fails the step instead of piping into bash (matches the existing curl -fL circuits fetch), and corrected the comment to name the logos-scaffold build step the workflow actually runs. Resolving.