Icon for NixOS + support

[dmadisetti]

Nix artwork can be found in https://github.com/NixOS/nixos-artwork

• white: https://github.com/NixOS/nixos-artwork/blob/master/logo/white.svg
• color: https://github.com/NixOS/nixos-artwork/blob/master/logo/nix-snowflake.svg

Additionally, I provided NixOS support to the install script: https://github.com/vinceliuice/grub2-themes/pull/158

• [x] Add icons
• [x] Allow for nix support (done! thanks! #158)

[dmadisetti]

I've been using this for awhile now. It's been great. Thanks (https://github.com/dmadisetti/.dots/blob/main/backgrounds/grub.jpg)

[dmadisetti]

Oh woah, didn't realize you already did it. Thanks!