add gigamonster256 repository

[gigamonster256]

The following points apply when adding a new repository to repos.json

• [x] I ran ./bin/nur format-manifest after updating repos.json (We will use the same script in github actions to make sure we keep the format consistent)
• [x] By including this repository in NUR I give permission to license the content under the MIT license.

Clarification where license should apply: The license above does not apply to the packages built by the Nix Packages collection, merely to the package descriptions (i.e., Nix expressions, build scripts, etc.). It also might not apply to patches included in Nixpkgs, which may be derivative works of the packages to which they apply. The aforementioned artifacts are all covered by the licenses of the respective packages.