Yarn update from 1.3.2 to 1.9.4

Author: ZoltanVeres

netlify.toml

@@ -2,5 +2,5 @@
   publish = "public"
   command = "npm run build"
 [build.environment]
-  YARN_VERSION = "1.3.2"
+  YARN_VERSION = "1.9.4"
   YARN_FLAGS = "--no-ignore-optional"