nix-output-monitor: Add zsh completion for nom-build

[maralorn]

Description of changes

I‘d be grateful if someone found similar solutions for bash and fish. Still, this is better than nothing.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 22.05 Release Notes (or backporting 21.11 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[mweinelt]

I wonder if that file should actually live inside the nix-output-monitor repository instead.

[maralorn]

I wondered about that as well, but currently nom-build itself does not live in that repo. And handing out completion files for an executable that we don‘t offer sounds weird.
I have thought about moving nom-build into the package, but the wrapper to put expect in the patch would be as large as the nom-build script is now.