The Authorization header was constructed as 'Basic ' + base64(token), which
GitHub rejects because Basic auth requires base64(username:password). The
malformed header caused all GitHub API requests to fail with HTTP 403,
which the fetch wrapper masks with a misleading 'you may be rate limited'
message.
This was tolerable on GitHub-hosted runners because their IPs aren't
aggressively rate limited unauthenticated, but on the new 1ES Azure-hosted
runners (shared outbound IPs) the 60 req/hr unauthenticated limit is hit
immediately, breaking core-ci on every PR.
Switch to the Bearer scheme, which is the correct format for both
GITHUB_TOKEN and PATs.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>