Use QC_GIT_TOKEN for gh auth in stubs repo discovery (#9432)

Continuation of #9430. The default GITHUB_TOKEN is scoped to the
workflow's own repo and can't see private QuantConnect/Lean.DataSource.*
repos, so gh repo list was returning only the public subset.
QC_GIT_TOKEN already has the org-level access required for cloning
private repos in ci_build_stubs.sh, so reuse it here for the discovery
step.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: AlexCatarino

.github/workflows/gh-actions.yml

@@ -31,7 +31,7 @@ jobs:
 
       - name: Compute additional stubs repos
         env:
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_TOKEN: ${{ secrets.QC_GIT_TOKEN || secrets.GITHUB_TOKEN }}
           ADDITIONAL_STUBS_REPOS: ${{ secrets.ADDITIONAL_STUBS_REPOS }}
         run: |
           REPOS=$(python find_datasource_repos.py)