refactor: RENOVATE_USER_AGENT

lib/platform/github/gh-got-wrapper.js

@@ -26,7 +26,9 @@ async function get(path, options, retries = 5) {
         {},
         {
           accept: appAccept,
-          'user-agent': 'https://github.com/renovatebot/renovate',
+          'user-agent':
+            process.env.RENOVATE_USER_AGENT ||
+            'https://github.com/renovatebot/renovate',
         },
         opts.headers
       );