fix(ci): fail the eval-test run when the suite does not pass (#350)

* fix(ci): fail the eval-test run when the suite does not pass

`nix eval .#evalTests` prints the suite result but exits 0 either way, so
both `just test` and the CI workflow reported success even when a test
failed. Compare the raw boolean and fail instead.

* fix(ci): use --json for the boolean eval result

`nix eval --raw` cannot coerce a Boolean to a string, and Just runs each
recipe line as a separate `nu -c`, so the result variable must stay on one
line.

.github/workflows/flake_evaltests.yml

@@ -37,6 +37,6 @@ jobs:
       - name: Run Nix Flake Eval Tests
         run: |
           echo 'Flake Eval Tests'
-          # stack overflow...
-          # nix eval .#checks --show-trace --print-build-logs --verbose
-          nix eval .#evalTests --show-trace --print-build-logs --verbose
+          # `nix eval` prints the suite result but still exits 0 on failure, so
+          # compare the JSON boolean explicitly to fail the job.
+          test "$(nix eval .#evalTests --json --show-trace)" = "true"