[DRE-maint] Bug#998097: ruby-eventmachine FTBFS on IPV6-only buildds and accesses the internet during the build

[Adrian Bunk]

Source: ruby-eventmachine
Severity: serious
Tags: ftbfs

FTBFS on IPV6-only buildds:

https://buildd.debian.org/status/fetch.php?pkg=ruby-eventmachine&arch=amd64&ver=1.3~pre20201020-b50c135-3&stamp=1633500476&raw=0
https://buildd.debian.org/status/fetch.php?pkg=ruby-eventmachine&arch=i386&ver=1.3~pre20201020-b50c135-4%2Bb1&stamp=1635578996&raw=0

FTBFS due to attempted DNS:

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/ruby-eventmachine.html

...
===============================================================================
Failure: test_nameserver(TestResolver):
  <nil> was expected to be kind_of?
  <String> but was
  <NilClass>.
/build/1st/ruby-eventmachine-1.3~pre20201020-b50c135/tests/test_resolver.rb:19:in `test_nameserver'
     16:   end
     17: 
     18:   def test_nameserver
  => 19:     assert_kind_of(String, EM::DNS::Resolver.nameserver)
     20:   end
     21: 
     22:   def test_nameservers
===============================================================================
...