chore: add RPC URL env var (#192)

## Change Summary

Add `OP_MAINNET_RPC_URL` env var.

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- [x] PR title adheres to the [conventional
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.changeset/thirty-geckos-study.md

@@ -0,0 +1,5 @@
+---
+"@farcaster/auth-relay": patch
+---
+
+chore: add RPC URL env var

apps/relay/src/env.ts

@@ -18,6 +18,7 @@ export const URL_BASE =
 export const HUB_URL = process.env["HUB_URL"] || "https://nemes.farcaster.xyz:2281";
 export const HUB_FALLBACK_URL = process.env["HUB_FALLBACK_URL"] || "https://hoyt.farcaster.xyz:2281";
 
-export const OPTIMISM_RPC_URL = process.env["OPTIMISM_RPC_URL"] || "https://mainnet.optimism.io";
+export const OPTIMISM_RPC_URL =
+  process.env["OPTIMISM_RPC_URL"] || process.env["OP_MAINNET_RPC_URL"] || "https://mainnet.optimism.io";
 
 export const AUTH_KEY = process.env["AUTH_KEY"];