Fix PR checks

At some point, `nix eval` stopped exiting on errors in nested objects.
Passing --json makes it fail if theres an error anywhere in the object.

.github/workflows/pr.yml

@@ -17,4 +17,4 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@master
-      - run: nix eval .#tests
+      - run: nix eval --json .#tests

.github/workflows/update.yml

@@ -21,5 +21,5 @@ jobs:
           git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
       - run: nix flake update --commit-lock-file
       - run: nix flake check --all-systems
-      - run: nix eval .#tests
+      - run: nix eval --json .#tests
       - run: git push