ci: pass NIX_CONFIG with GitHub token into all test containers

- Add NIX_CONFIG with GitHub access token to all CI test workflows
- Export NIX_CONFIG in Makefile for propagation to test scripts
- Forward NIX_CONFIG explicitly into all docker run invocations
- Prevent GitHub API rate limit errors during Nix-based tests

https://chatgpt.com/share/69432655-a948-800f-8c0d-353921cdf644

.github/workflows/test-integration.yml

@@ -7,7 +7,9 @@ jobs:
   test-integration:
     runs-on: ubuntu-latest
     timeout-minutes: 30
-
+    env:
+      NIX_CONFIG: |
+        access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4