Buildkite agent fixup

[mrkkrp]

Motivation for this change

There are two changes:

• Update the buildkite agent version.
• Fix a bug in NixOS' buildkite agent service module so that the services.buildkite-agent.package actually has effect. In addition to that, version 3 allows us to set tags="gueue=test" which is rejected if meta-data is set before tags the whole configuration is rejected. So I figured that we should only set meta-data when it's actually specified by the user.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[grahamc]

Another improvement which would be nice to make is letting openssh.privateKeyPath and openssh.publicKeyPath be optional.

[mrkkrp]

I have to go now, but if you can wait a little bit I'll make those optional. And maybe add more options that we actually use instead of putting them in extra config.

[grahamc]

How about we do that as a different PR?

A third thing which would be really nice is making metadata an attribute set:

metadata = {
    queue = "test";
    another-thing = "whatever";
};

I never remember how to set metadata, and get it wrong. If it was an attribute set like this, it would be immediately obvious.

[mrkkrp]

I’m OK with doing it in a different PR. In that case you can merge this one, unless there is something else to be done?

[grahamc]

I forgot to test that building buildkite worked, and so I tested it locally and it did :)