k3s: build with go_1_20

[euank]

Description of changes

k3s upstream does not support go 1.21, so we're stuck on 1.20 for now.
See the linked issue below for more info on that!

Fixes #263580

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)

    $ nix build '.#k3s.passthru.tests.single-node' 
    $ nix build '.#k3s_1_26.passthru.tests.single-node' 
    $ nix build '.#k3s_1_25.passthru.tests.single-node' 
    

• 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.