feat(build): skip already-built package versions on workflow restart (#91)

## Summary

When a `build-all-*` workflow is restarted, the build job re-reads the static `pkgs_to_build.rds` that the install-deps step produced once, so it cycles over every package an interrupted run already built. This adds a DB-based skip filter so a restart only processes what is genuinely left.

- At job start, `build-all.R` queries the `single_builds` metadata table for `(name, tag)` already built successfully (`error_occurred = FALSE`) on this `platform`/`arch`, and drops those pairs from the chunk before the build loop. It logs how many it skipped.
- One indexed query, one round trip, run before the pak forks — no extra S3 listing and no new Python/s3fs memory pressure (`RPostgres`/`DBI` are already used in the container).
- Errored versions are intentionally **not** skipped, so transient failures still get retried on restart.

## Dependency

Correctness depends on a `error_occurred = FALSE` row meaning the binary is actually published. That guarantee is added in rpkgs/bincraft#56 (success row written only after a confirmed S3 upload). This PR should land together with / after a bincraft release including that fix.

Reviewed-on: #91

local/build-all.R

@@ -39,6 +39,31 @@ sprintf("# of package versions for this job: %s", nrow(chunk))
 exclude <- jsonlite::fromJSON("local/excluded-packages.json")[["package"]]
 chunk <- chunk[!chunk$Package %in% exclude, ]
 
+# Skip package versions already built in a previous run.
+# pkgs_to_build.rds is a static snapshot from the install-deps step, so on a
+# restart it still lists everything an interrupted run already produced. The
+# metadata DB reflects that progress, so we re-derive the remaining set here.
+platform <- paste(Sys.getenv("OS"), gsub("[.]", "", Sys.getenv("OS_VERSION")), sep = "-")
+arch <- Sys.getenv("ARCH")
+con <- DBI::dbConnect(
+  RPostgres::Postgres(),
+  dbname = "build_metadata",
+  host = "r-binaries.devxy.io",
+  port = 15432,
+  user = "rpkgs",
+  password = Sys.getenv("PGPASS"),
+  sslmode = "require"
+)
+built <- DBI::dbGetQuery(
+  con,
+  "SELECT name, tag FROM single_builds WHERE platform = $1 AND arch = $2 AND error_occurred = FALSE",
+  params = list(platform, arch)
+)
+DBI::dbDisconnect(con)
+before <- nrow(chunk)
+chunk <- chunk[!paste(chunk$Package, chunk$Version) %in% paste(built$name, built$tag), ]
+sprintf("Skipped %d already-built package versions; %d remaining for this job", before - nrow(chunk), nrow(chunk))
+
 # Read pre-computed S3 listing from install-deps step
 # This avoids loading s3fs/reticulate/Python in the build container,
 # saving significant memory for pak subprocess forks