Thanks, @indirect. I have done lots of rebuilds on this machine, but only a couple hundred requests per manually started build (which look normal and expected to both you and me).
Could some bad actor on the same network be the problem? (I know that I had this problem on another machine, but I can’t remember now where that machine was).
Here are the 429s I got on that same build:
HTTP 429 Too Many Requests https://index.rubygems.org/info/fast_blank
HTTP 429 Too Many Requests https://index.rubygems.org/info/ember-rails
HTTP 429 Too Many Requests https://index.rubygems.org/info/message_bus
HTTP 429 Too Many Requests https://index.rubygems.org/info/maxminddb
HTTP 429 Too Many Requests https://index.rubygems.org/info/mock_redis
HTTP 429 Too Many Requests https://index.rubygems.org/info/mini_scheduler
HTTP 429 Too Many Requests https://index.rubygems.org/info/oauth
HTTP 429 Too Many Requests https://index.rubygems.org/info/omniauth-instagram
HTTP 429 Too Many Requests https://index.rubygems.org/info/rack-openid
HTTP 429 Too Many Requests https://index.rubygems.org/info/omniauth-openid
HTTP 429 Too Many Requests https://index.rubygems.org/info/openid-redis-store
HTTP 429 Too Many Requests https://index.rubygems.org/info/parser
HTTP 429 Too Many Requests https://index.rubygems.org/info/email_reply_trimmer
HTTP 429 Too Many Requests https://index.rubygems.org/info/htmlentities
HTTP 429 Too Many Requests https://index.rubygems.org/info/guess_html_encoding
HTTP 429 Too Many Requests https://index.rubygems.org/info/logster
HTTP 429 Too Many Requests https://index.rubygems.org/info/http_accept_language
HTTP 429 Too Many Requests https://index.rubygems.org/info/memory_profiler
HTTP 429 Too Many Requests https://index.rubygems.org/info/mocha
HTTP 429 Too Many Requests https://index.rubygems.org/info/omniauth-github
HTTP 429 Too Many Requests https://index.rubygems.org/info/ember-data-source
HTTP 429 Too Many Requests https://index.rubygems.org/info/fast_xs
HTTP 429 Too Many Requests https://index.rubygems.org/info/moneta
HTTP 429 Too Many Requests https://index.rubygems.org/info/omniauth-oauth
HTTP 429 Too Many Requests https://index.rubygems.org/info/mustache
HTTP 429 Too Many Requests https://index.rubygems.org/info/rinku
HTTP 429 Too Many Requests https://index.rubygems.org/info/sassc
HTTP 429 Too Many Requests https://index.rubygems.org/info/rtlit
HTTP 429 Too Many Requests https://index.rubygems.org/info/ruby-readability
HTTP 429 Too Many Requests https://index.rubygems.org/info/seed-fu
HTTP 429 Too Many Requests https://index.rubygems.org/info/shoulda-context
HTTP 429 Too Many Requests https://index.rubygems.org/info/shoulda-matchers
HTTP 429 Too Many Requests https://index.rubygems.org/info/webpush
HTTP 429 Too Many Requests https://index.rubygems.org/info/webmock