[devbox] GHA tests: add GITHUB_TOKEN to nix config to fix 403 errors

Author: savil

.github/workflows/cli-tests.yaml

@@ -48,7 +48,9 @@ env:
   HOMEBREW_NO_EMOJI: 1
   HOMEBREW_NO_ENV_HINTS: 1
   HOMEBREW_NO_INSTALL_CLEANUP: 1
-
+  NIX_CONFIG: |
+    access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+    
 jobs:
   build-devbox:
     strategy: