SCCache + unify public/private builds

[graydon]

This continues from #5192 (enabling namespace-backed shared SCCache in the build) to also merge the public and private builds, using namespace for both. It also updates the mac build path a bit to be more like the linux one and bumps the version of rustup we're using to something less-incredibly-old (which runs faster and more reliably).

[Copilot]

Pull request overview

This PR continues the CI caching work by consolidating public/private CI onto the Namespace-backed runner setup, enabling Namespace-configured sccache usage, and modernizing Rust toolchain installation (including macOS/arm64 support).

Changes:

• Unifies CI workflows by removing the separate private build workflow and updating the main build workflow to run on Namespace runners for both public/private repos.
• Updates Rust installation to a newer pinned rustup-init and adds host-triple handling for Darwin/arm64.
• Refactors CI build script to drop ccache setup, optionally enable --enable-nsc-sccache, and add cargo/rustup cache purging logic.

Reviewed changes

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

File	Description
install-rust.sh	Bumps pinned rustup-init version, adds Darwin/arm64 host support, tweaks checksum verification and installer flags.
ci-build.sh	Removes “skip if already tested” and ccache handling; adds cargo/rustup cache purging and conditional --enable-nsc-sccache.
.github/workflows/build.yml	Switches jobs to Namespace setup with OIDC permissions, adjusts caching to track last-tested commit, installs sccache, and unifies behavior across repo visibilities.
.github/workflows/build-private.yml	Deletes the now-redundant private-only workflow.