feat(server): Cache built manifests to the GCS bucket

Caches manifests under `manifests/$cacheKey` in the GCS bucket and
introduces two-tiered retrieval of manifests from the caches (local
first, bucket second).

There is some cleanup to be done in this code, but the initial version
works.

tools/nixery/server/main.go

@@ -125,7 +125,7 @@ type registryHandler struct {
 	cfg    *config.Config
 	ctx    *context.Context
 	bucket *storage.BucketHandle
-	cache  *builder.BuildCache
+	cache  *builder.LocalCache
 }
 
 func (h *registryHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {