fix: save S3 listing in precompute step, avoid loading s3fs in build step

s3fs uses reticulate/Python which significantly increases R process memory.
When pak tries to fork R to install package dependencies, the enlarged
process can't be duplicated within the container memory limit.

Now the install-deps step saves the S3 file listing as s3_cache.rds,
and the build step reads it with readRDS() — no s3fs loading needed.

local/build-all.R

@@ -40,21 +40,10 @@ exclude <- c(
 )
 chunk <- chunk[!chunk$Package %in% exclude, ]
 
-# Bulk S3 listing upfront — avoids per-package S3 calls
-s3fs::s3_file_system(
-  aws_access_key_id = Sys.getenv("B2_S3_ACCESS_KEY"),
-  aws_secret_access_key = Sys.getenv("B2_S3_SECRET_KEY"),
-  endpoint = "https://s3.eu-central-003.backblazeb2.com",
-  region_name = "eu-central-003",
-  refresh = TRUE
-)
-codename <- bincraft::set_codename(NULL)
-local_arch <- Sys.info()[["machine"]]
-arch <- if (grepl("arm64|aarch64", local_arch)) "arm64" else "amd64"
-s3_cache <- basename(s3fs::s3_dir_ls(
-  sprintf("devxy-rpkgs-binaries/%s/%s/latest/src/contrib", arch, codename),
-  recurse = TRUE
-))
+# 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
+s3_cache <- readRDS("/mnt/cache/packages/s3_cache.rds")
 sprintf("S3 cache: %s files", length(s3_cache))
 
 n <- nrow(chunk)