bind: 9.12.3-P4 -> 9.12.4-P1

[dtzWill]

Motivation for this change

https://www.openwall.com/lists/oss-security/2019/04/25/1

CVE-2018-5743, CVE-2019-6467, and CVE-2019-6468

• backport
• security

We may want to move to 9.14.1 since 9.12 EOL was May apparently,
but since patches didn't apply and it's late I'll leave that
for another time or someone else :).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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Let's make sure this builds on aarch (or at least doesn't newly fail)
since the linked thread (see commit) has someone mentioning an issue.