add ryangibb as trusted user

[RyanGibb]

• I have read the entire README https://github.com/nix-community/aarch64-build-box
• I completely understood the README https://github.com/nix-community/aarch64-build-box
• I know when I can't trust the builder, as explained in the README

[RyanGibb]

This would be quite useful in building an image for the Raspberry Pi 5 NixOS/nixpkgs#260754 (comment)

[RyanGibb]

Putting a gentle ping here, @grahamc.

From the README:

I'll grant access to well known members of the community, and people well known members in the community trust.

I don't know whether I constitute a well known member of the community, but I've been contributing to Nixpkgs for 2 years, am maintaining a couple of Nix/NixOS projects, and I attended the FOSDEM Nix and NixOS devroom this year https://ryan.freumh.org/blog/fosdem/#nix-and-nixos.