ext/io/console/io-console.gemspec was embedding a timestamp which made
the build not reproducible. Gems respect SOURCE_DATE_EPOCH so it's
enough to just delete that line if it exists.
This file has been fixed in
679a941d05 (diff-d8422f096931c58d4463e2489f62a228b0f24f0492950ba88c8c89a0d741cfe6)
And then ruby regularly merges that gem into their own repository. Ruby
master is fixed but none of the ruby releases have been fixed yet.
lib/ruby/gems/2.6.0/specifications/default/io-console-0.4.7.gemspec now
contains:
s.date = "1980-01-01"