Win release host's wrangler OAuth is logged into a different Cloudflare
account than the R2-enabled one, so `wrangler r2 bucket info` returns
"Please enable R2 through the Cloudflare Dashboard" and release-local-winlinux
fails before any upload. The S3 API credentials (R2_ACCESS_KEY_ID /
R2_SECRET_ACCESS_KEY) carry their own account scope and are the path
we already use for every upload and purge, so when they are set we can
short-circuit the wrangler bucket-info/create and dev-url dance entirely.
Also requires R2_PUBLIC_BASE_URL in .env.release.local when S3 mode is
on, since the wrangler-based dev-url fallback is unreachable in that
mode.
ZacharyZhang-NY