Speed up Rust and C++ CI builds with sccache and a remote S3 backend

Steps

1. Install sccache on the CI runner: cargo install sccache or download a prebuilt binary from the sccache GitHub releases page and add it to PATH
2. Set environment variables to activate sccache for Rust: RUSTC_WRAPPER=sccache; for C/C++, set CC='sccache gcc' and CXX='sccache g++'
3. Configure the S3 backend by setting SCCACHE_BUCKET to your bucket name, SCCACHE_REGION to the AWS region, and providing AWS credentials via the standard AWS_ACCESS_KEY_ID/AWS_SECRET_ACCESS_KEY variables or an IAM role
4. Add SCCACHE_S3_USE_SSL=true and optionally SCCACHE_S3_SERVER_SIDE_ENCRYPTION=true for encrypted objects; for S3-compatible stores, set SCCACHE_S3_ENDPOINT to your endpoint URL
5. Verify cache activity by running sccache --show-stats at the end of the CI job; the output shows compile requests, cache hits, and misses per language
6. Cache the sccache binary itself (or use a fixed version in PATH) to avoid reinstalling it on every CI run; the compiled artifacts in S3 are the primary cache, not the binary