ci: deploy in slow mode

Transactions were prone to getting stuck in the mempool, for some reason,
when sending a lot of them in parallel. It would be worth investigating
why, but in the meantime, just stick to one TX at a time. Hopefully, this
helps.

contracts/utils/deploy-cli.ts

@@ -107,6 +107,7 @@ export async function main() {
  "--rpc-url",
  options.rpcUrl,
  "--broadcast",
+ "--slow",
  ];
 
  // verify