r-lib/actions/setup-r@v2 runs `sudo apt-get update` before installing R, which fails outright if any configured apt source is broken - even one we never touch. The google-chrome source baked into GitHub's ubuntu-latest image intermittently serves a stale Packages.gz that doesn't match its own Release file's hash (a Google CDN sync race), which took down every Linux leg of the R-CMD-check matrix on 2026-09-09 regardless of R version. Removing the source we don't need makes this failure class impossible here. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>