fix remaining niv reference

Author: nrdxp

.buildkite/pipeline.yml

@@ -1,7 +1,7 @@
 steps:
   - label: ":nix::point_right::pipeline:"
     command: |
-      export NIX_PATH="nixpkgs=$(nix-instantiate --eval --strict --json --read-write-mode -E '(import nix/sources.nix).nixpkgs' | tr -d '"')"
+      export NIX_PATH="nixpkgs=$(nix-instantiate --eval --strict --json --read-write-mode -E '(import nix/compat.nix).defaultNix.inputs.nixpkgs' | tr -d '"')"
       nix-instantiate --eval --strict --json --expr '(import ./.buildkite { pipeline = ./.buildkite/pipeline.nix; })' \
       | buildkite-agent pipeline upload --no-interpolation
     agents: