fix missing variable

index.js

@@ -3,6 +3,7 @@ const core = require('@actions/core');
 const http = require('http');
 const https = require('https');
 
+const req_timeout_ms = 30_000;
 const manifest_media_type = 'application/vnd.docker.distribution.manifest.v2+json';
 
 main();
@@ -101,7 +102,7 @@ async function http_req(method, url_str, headers, body) {
 			});
 		});
 	
-		req.setTimeout(timeout, () => {
+		req.setTimeout(req_timeout_ms, () => {
 			req.destroy();
 			reject(new Error('request timeout'));
 		});