fix prepare script (#163)

Author: zjb0807

.github/workflows/publish.yml

@@ -31,7 +31,7 @@ jobs:
           if [ $(yarn npm info '@acala-network/contracts' --json | jq -er ".versions|any(. == $version)") == false ]
           then
               # prepare
-              yarn prepare
+              yarn workspace '@acala-network/contracts' prepare
               # publish
               yarn tsc -p tsconfig-cjs.json && yarn workspace '@acala-network/contracts' npm publish --access=public
           fi