fix: make relayer work again

Fixes a bug where the relayer package was not correctly running the
start script since the start script updated recently.

.changeset/kind-zoos-tell.md

+---
+'@eth-optimism/message-relayer': patch
+---
+
+Fixes a bug that prevented the relayer from running correctly.

ops/scripts/relayer.sh

@@ -23,4 +23,4 @@ curl \
     $L2_NODE_WEB3_URL
 
 # go
-exec node ./exec/run-message-relayer.js
+exec yarn start

packages/message-relayer/src/exec/run.ts

@@ -117,4 +117,5 @@ const main = async () => {
 
   await service.start()
 }
-export default main
+
+main()