Don't AppendDescriptor until we've written config (ko-build#1175)

I have a suspicion that this is causing a race where we modify the
cache's index.json file before we actually write out the image's config
file, so if we call RawConfigFile before we finish caching it, we'll
just fail to find that file.

Signed-off-by: Jon Johnson <jon.johnson@chainguard.dev>

pkg/commands/cache.go

@@ -165,10 +165,6 @@ func (i *imageCache) get(ctx context.Context, ref name.Reference, missFunc baseF
 			return result, err
 		}
 
-		if err := i.p.AppendDescriptor(*desc); err != nil {
-			return result, err
-		}
-
 		if _, ok := result.(v1.ImageIndex); ok {
 			result = &lazyIndex{
 				ref:      ref,
@@ -202,6 +198,10 @@ func (i *imageCache) get(ctx context.Context, ref name.Reference, missFunc baseF
 				cache:    i,
 			}
 		}
+
+		if err := i.p.AppendDescriptor(*desc); err != nil {
+			return result, err
+		}
 	}
 
 	i.cache.Store(ref.String(), result)