ci: build riscv64

[Xeonacid]

Unlike aarch64, the riscv64 sccache binary is not static, so use the host architecture one. 1

[Copilot]

Pull request overview

Adds CI coverage and build configuration for riscv64 (Linux) builds, including sysroot setup and workflow matrix expansion. It also adjusts sccache handling to avoid using a non-static riscv64 sccache binary by falling back to the host architecture binary.

Changes:

• Add GN args + sysroot installation for riscv64 targets in build.rs.
• Extend GitHub Actions CI matrix to build/test riscv64gc-unknown-linux-gnu (debug/release, with and without simdutf).
• Add a riscv64 cross-compilation toolchain install step and adjust sccache platform selection for riscv64.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
build.rs	Adds riscv64 target CPU + sysroot install logic for V8 GN generation.
.github/workflows/ci.yml	Adds riscv64 CI jobs, installs riscv64 cross toolchain/QEMU, and maps riscv64 to host sccache binary.

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[Copilot]

The sccache version is bumped to v0.14.0 for the main build job, but other jobs in this workflow still pin sccache to v0.8.2. If the version bump is intentional, consider updating the other sccache install steps (or centralizing the version into a single env/variable) to avoid inconsistent behavior and harder-to-debug CI issues across jobs.