Some hosts resolve go.dev/dl through an IPv6 path that serves the site and
manifest fine but returns 404 for the tarball CDN (dl.google.com). curl
prefers IPv6 by default, so ensure_go's download loop failed on every
candidate and aborted the local build.
- add curl_retry: fetch with the default address family, fall back to
'curl -4' on failure; use it for both the manifest and the tarball
- the IPv4 fallback is a pure retry — hosts with working IPv6 are
unaffected, and pure-IPv4 hosts were never broken
- drop an unused 'latest' variable in the download loop
bluebluesoda
f2d3eee4d8