Fix passing KUBECONFIG to nix

Author: lukaszcl

.github/workflows/k8s-e2e.yaml

@@ -90,7 +90,7 @@ jobs:
           cl_image_tag: ${{ env.CHAINLINK_VERSION }}
           test_command_to_run: |
             unset ENV_JOB_IMAGE
-            nix develop --keep KUBECONFIG -c sh -c "cd lib && make k8s_test_e2e_ci"
+            nix develop -c sh -c "export KUBECONFIG=${KUBECONFIG} && cd lib && make k8s_test_e2e_ci"
           artifacts_location: ./e2e/logs
           publish_check_name: E2E Test Results
           token: ${{ secrets.GITHUB_TOKEN }}