workers: default body to provided payload

This was previously incorrectly always using `body` (an uninitialised
slice of bytes) for the script payload when the configuration was not
for an ES module upload. Fixes the upload so that both types are
covered.

.changelog/1155.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+workers: correctly set `body` value for non-ES module uploads
+```

workers.go

@@ -214,10 +214,10 @@ func (api *API) UploadWorker(ctx context.Context, rc *ResourceContainer, params
 		return WorkerScriptResponse{}, ErrMissingAccountID
 	}
 
+	body := []byte(params.Script)
 	var (
 		contentType = "application/javascript"
 		err         error
-		body        []byte
 	)
 
 	if params.Module || len(params.Bindings) > 0 {