worker: introduce job artifact directory

The `jobs/:job_id/builds/:build_id/image` route was awkward: the
`:jobid` was actually weldr's compose id and `:build_id` was always `0`.

Change it to `jobs/:job_id/artifacts/:name`, where `:job_id` is now a
job id, and `:name` is the name of the artifact to upload. In the
future, it could support uploading more than one artifact.

This allows removing outputs from `store`, which is now back to being a
pure JSON-store. Take care that `weldr` returns (and deletes) images
from the new (or for backwards compatibility, the old) location.

The `org.osbuild.local` target continues to exist as a marker for the
worker to know whether it should upload artifacts.

internal/worker/client.go

@@ -118,9 +118,8 @@ func (c *Client) UpdateJob(job *Job, status common.ImageBuildState, result *comm
 	return nil
 }
 
-func (c *Client) UploadImage(composeId uuid.UUID, imageBuildId int, reader io.Reader) error {
-	// content type doesn't really matter
-	url := c.createURL(fmt.Sprintf("/job-queue/v1/jobs/%s/builds/%d/image", composeId, imageBuildId))
+func (c *Client) UploadImage(job uuid.UUID, name string, reader io.Reader) error {
+	url := c.createURL(fmt.Sprintf("/job-queue/v1/jobs/%s/artifacts/%s", job, name))
 	_, err := c.client.Post(url, "application/octet-stream", reader)
 
 	return err