as an alternative, you may add `1s` after `github` in your address bar to open the repository with a browser-based VS Code, and then use Ctrl + Shift + F to search across all files
EDIT: No, I was wrong, you can search it without authentication.
But that is repository search only, of course and not github-wide.
https://docs.github.com/en/codespaces/the-githubdev-web-base...