refactor: single source of truth for excluded packages

local/build-all.R had a 75-name hardcoded vector that drifted from
local/excluded-packages.json (RcmdrPlugin.ROC was in the R vector
but missing from the JSON). weekly-rebuild-missing-* already reads
the JSON, so this aligns build-all.R with that pattern.

- Add the missing RcmdrPlugin.ROC entry to excluded-packages.json
- Replace the hardcoded vector in build-all.R with
  jsonlite::fromJSON("local/excluded-packages.json")[["package"]]
- Pull jsonlite into the install-deps step so it's available in
  /mnt/cache/R-pkgs for the build step
- Drop the dead base64 SKIP_PKGS docs comment from
  build-all-versions-arm64.yaml; point to the JSON instead

.crow/build-all-versions-install-deps-amd64.yaml

@@ -34,7 +34,7 @@ steps:
       - mkdir -p /mnt/cache/pkgcache /mnt/cache/R-pkgs /mnt/cache/ccache /mnt/cache/packages
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
       - git clone -q https://codefloe.com/rpkgs/bincraft.git /tmp/bincraft
-      - /opt/R/$R_VERSION/bin/R -q -e 'pak::sysreqs_db_update(); pak::local_install("/tmp/bincraft"); pak::pak(c("RPostgres", "s3fs", "data.table", "future")); packageVersion("bincraft")'
+      - /opt/R/$R_VERSION/bin/R -q -e 'pak::sysreqs_db_update(); pak::local_install("/tmp/bincraft"); pak::pak(c("RPostgres", "s3fs", "data.table", "future", "jsonlite")); packageVersion("bincraft")'
       - /opt/R/$R_VERSION/bin/R -q -e "source('local/packages-to-build.R'); saveRDS(pkgs, '/mnt/cache/packages/pkgs_to_build.rds'); sprintf('Precomputed %s package versions to build', nrow(pkgs))"
     backend_options:
       docker: