Fix wrong URL in getTargetUrl (#677)

## Summary
Fix wrong URL in `getTargetUrl` when `this.apiBaseUrl` is set. This can
happen when `INNGEST_DEV=1`.

## Checklist
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- [x] Added changesets if applicable

.changeset/dirty-bats-pay.md

@@ -0,0 +1,5 @@
+---
+"inngest": patch
+---
+
+Fix wrong URL when INNGEST_DEV=1

packages/inngest/src/api/api.ts

@@ -78,6 +78,10 @@ export class InngestApi {
   }
 
   private async getTargetUrl(path: string): Promise<URL> {
+    if (this.apiBaseUrl) {
+      return new URL(path, this.apiBaseUrl);
+    }
+
     let url = new URL(path, defaultInngestApiBaseUrl);
 
     if (this.mode.isDev && this.mode.isInferred && !this.apiBaseUrl) {