[FIX] Align nix expression evaluation with recent changes (#906)

* [FIX] Align nix expression evaluation with recent changes

* [FIX] Update .buildkite/pipeline.yml

Co-authored-by: Silvan Mosberger <[email protected]>

Co-authored-by: Silvan Mosberger <[email protected]>

Author: dzajkowski

.buildkite/pipeline.yml

@@ -1,8 +1,8 @@
 steps:
   - label: ":nix::point_right::pipeline:"
     command: |
-      export NIX_PATH="nixpkgs=$(nix eval --raw '(import nix/sources.nix).nixpkgs')"
-      nix eval --json '(import ./.buildkite { pipeline = ./.buildkite/pipeline.nix; })' \
+      export NIX_PATH="nixpkgs=$(nix-instantiate --eval --strict --json --read-write-mode -E '(import nix/sources.nix).nixpkgs' | tr -d '"')"
+      nix-instantiate --eval --strict --json --expr '(import ./.buildkite { pipeline = ./.buildkite/pipeline.nix; })' \
       | buildkite-agent pipeline upload --no-interpolation
     agents:
       queue: project42