Answer held and say artifacts-gone, so the builder rebuilds a done key whose blob the artifact store no longer holds (issue 1937)

A done key the controller never took in holds no manifest, and the store's nightly collector deleted its bare blob:
node-tools', mesh-host's and the forge bridge's keys name blobs that answer 404. Since issue 1933 the take-in refuses such a
key, and nothing built it again. held (in the serving process, HEADs only) lets the builder ask before it reuses a key; a
refused take-in now names every gone archive and says artifacts-gone with the module, key, fingerprint and references.

module.json

@@ -102,6 +102,7 @@
     "data",
     "build",
     "artifacts",
+    "held",
     "collect",
     "images",
     "mirrors",