butler: build with go_1_20

[SuperSamus]

Description of changes

Build failing since #254878.
Reported upstream on itchio/butler#256.

Some notes that are out of scope:
Itch itself has also regressed from the latest staging, and it still doesn't work:

[2420:1026/115924.099013:FATAL:gpu_data_manager_impl_private.cc(445)] GPU process isn't usable. Goodbye.

#248321 fixes it, but... it's using a beta.
(And --disable-gpu-sandbox still isn't enough, the application opens, but it's just a black screen)

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.