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

scripts/test/test-env-virtual.sh

@@ -19,6 +19,7 @@ if OUTPUT=$(docker run --rm \
     -e REINSTALL_PKGMGR=1 \
     -v "$(pwd):/opt/src/pkgmgr" \
     -w /opt/src/pkgmgr \
+    -e NIX_CONFIG="${NIX_CONFIG}" \
     "${IMAGE}" \
     bash -lc '
         set -euo pipefail