accounts/keystore: assign schema as const instead of var (#16985)

accounts/keystore/keystore.go

@@ -50,7 +50,7 @@ var (
 var KeyStoreType = reflect.TypeOf(&KeyStore{})
 
 // KeyStoreScheme is the protocol scheme prefixing account and wallet URLs.
-var KeyStoreScheme = "keystore"
+const KeyStoreScheme = "keystore"
 
 // Maximum time between wallet refreshes (if filesystem notifications don't work).
 const walletRefreshCycle = 3 * time.Second