refactor: single source of truth for excluded packages (#79)

## Summary

`local/build-all.R` had a 75-name hardcoded `exclude <- c(...)` vector that had drifted from `local/excluded-packages.json` — `RcmdrPlugin.ROC` was in the R vector but missing from the JSON.

`.crow/weekly-rebuild-missing-*.yaml` already reads the JSON via `jsonlite::fromJSON(...)[["package"]]`. This brings `build-all.R` in line with that pattern.

Changes:

- **`local/excluded-packages.json`**: add the missing `RcmdrPlugin.ROC` entry (reason `"hang"`, matching siblings).
- **`local/build-all.R`**: replace the 16-line hardcoded vector with one `jsonlite::fromJSON(...)` call.
- **`.crow/build-all-versions-install-deps-{amd,arm}64.yaml`**: add `jsonlite` to the install-deps `pak::pak()` list so it's available in `/mnt/cache/R-pkgs` for the build step.
- **`.crow/build-all-versions-arm64.yaml`**: drop the dead base64-encoded `SKIP_PKGS` docs comment that nobody was passing as a `--var` anyway; replace with a one-line pointer to the JSON.

Reviewed-on: #79

.crow/build-all-versions-install-deps-arm64.yaml

@@ -33,7 +33,7 @@ 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: