ci: update workflow

Author: Matthieu Coudron

.github/workflows/default.yml

@@ -7,8 +7,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v18
+    - uses: cachix/install-nix-action@v21
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - run: |
-        nix develop ./contrib -c make test
+        nix develop . -c make test