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

local/excluded-packages.json

@@ -61,6 +61,7 @@
   {"package": "RcmdrPlugin.DoE", "reason": "hang"},
   {"package": "RcmdrPlugin.NMBU", "reason": "hang"},
   {"package": "RcmdrPlugin.RiskDemo", "reason": "hang"},
+  {"package": "RcmdrPlugin.ROC", "reason": "hang"},
   {"package": "RcmdrPlugin.TeachStat", "reason": "hang"},
   {"package": "RcmdrPlugin.TeachingDemos", "reason": "hang"},
   {"package": "RcmdrPlugin.UCA", "reason": "hang"},