fix(producer): list correct collection for public keys

packages/producer/mod.ts

···
     while (true) {
       const res = await this.rpc.get("com.atproto.repo.listRecords", {
         params: {
-          collection: "app.cistern.lexicon.item",
+          collection: "app.cistern.lexicon.pubkey",
           repo: this.did,
           cursor,
         },