Start using sccache in CI

[graydon]

This switches us over from using cache volumes for build artifacts to using sccache for build artifacts. We still use cache volumes for ~/.cargo contents but that ought not to interfere with anything.

Rationale:

• Using cache volumes is possibly more-expensive than sccache? (maybe? or at least billed at coarser granularity?)
• Sccache will give better cache hit rates across all builds (including PRs and even dev workstation builds if you use it right)
• Cache volume management logic is a bit fragile on its own, we get to delete a bunch of code
• Most importantly: using cache volumes amplifies a bug in Makefile.am to into a cross-CI-session artifact reuse; sccache is a more trustworthy source of cache-logic than our own Makefile hacking (this has happened: see Fix stale Soroban host rebuild invalidation. #5187)

[Copilot]

Pull request overview

This PR switches CI build artifact caching from Namespace cache volumes (persisting build directories) to remote compilation caching via sccache (configured through nsc), while still caching ~/.cargo contents.

Changes:

• Update ci-build.sh to configure builds with --enable-nsc-sccache and remove ccache/cargo sweep logic.
• Update GitHub Actions workflows to authenticate to Namespace Cloud, install sccache, and stop caching build output directories.
• Move “already tested commit” marker to a home-directory file that can be cached independently of build directories.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
ci-build.sh	Switch build configuration from ccache to nsc-configured sccache and simplify cache purging/reporting.
.github/workflows/build.yml	Add Namespace Cloud auth/permissions, install sccache, adjust cached paths, and update “last tested commit” storage.