artifact-path implementation

[clbx]

Is your feature request related to a problem? Please describe.
The artifact-path argument in buildkite does not work since it runs on the agent, not the job.

Describe the solution you'd like
An argument for the plugin that replicates the features of the buildkite argument.

Additional context
Ive thought about when this could be done, the command could be appended to the end of any commands run in the pod, or it may be possible in a post-command step, but I'm not sure when this could be done.

I plan on opening a PR for this in the future when I get time, but I don't have the capacity right now.

[tgolsson]

The plugin runs in the container aswell but currently just fizzles; might be a good place to hook there? Should have access to all the configuration iirc.

k8s-buildkite-plugin/hooks/pre-exit

Lines 9 to 12 in 720b4b7

	if [[ -v "BUILDKITE_PLUGIN_K8S_IS_JOB" ]]; then
	# do not recurse
	exit 0
	fi

[clbx]

Yeah I think that would be a good place to put it.