fix: add baseUrl to Octokit constructor, if appropriate, for GHE (#52)

This attempts to fix the issue reported at
https://github.com/openai/codex-action/issues/32.

I don't have access to a GHE account, so this is hard to test, but this
looks right according to:


https://github.com/octokit/octokit.js/blob/45c56ffaa6d1799dd4ebaf83f06a8fc64fc39c49/README.md#L197-L201

src/checkActorPermissions.ts

@@ -103,7 +103,13 @@ export async function ensureActorHasWriteAccess(
     };
   }
 
-  const octokit = options.octokit ?? new Octokit({ auth: token });
+  const baseUrl = process.env.GITHUB_API_URL?.trim();
+  const octokit =
+    options.octokit ??
+    new Octokit({
+      auth: token,
+      ...(baseUrl ? { baseUrl } : {}),
+    });
 
   core.info(`Checking write access for actor '${actor}' on ${owner}/${repo}`);