Motivation for this change

The upstream release tag (https://github.com/intel/Intel-Linux-Processor-Microcode-Data-Files/releases/tag/microcode-20190618) doesn't feature release notes, but e.g. Debian has a changelog entry that reads like this:

intel-microcode (3.20190618.1) unstable; urgency=medium

  * New upstream microcode datafile 20190618
    + SECURITY UPDATE
      Implements MDS mitigation (RIDL, Fallout, Zombieload), INTEL-SA-00223
      CVE-2018-12126, CVE-2018-12127, CVE-2018-12130, CVE-2019-11091
      for Sandybridge server and Core-X processors
    + Updated Microcodes:
      sig 0x000206d6, pf_mask 0x6d, 2019-05-21, rev 0x061f, size 18432
      sig 0x000206d7, pf_mask 0x6d, 2019-05-21, rev 0x0718, size 19456
  * Add some missing (minor) changelog entries to 3.20190514.1
  * Reformat 3.20190514.1 changelog entry to match rest of changelog

 -- Henrique de Moraes Holschuh <hmh@debian.org>  Wed, 19 Jun 2019 09:05:54 -0300

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.