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.

Justfile

@@ -21,7 +21,7 @@ default:
 # Run eval tests
 [group('nix')]
 test:
-  nix eval .#evalTests --show-trace --print-build-logs --verbose
+  let result = (nix eval .#evalTests --json --show-trace --print-build-logs --verbose | str trim); if $result != "true" { error make { msg: $"eval tests failed: evalTests returned ($result)" } }
 
 # Evaluate a NixOS host configuration without building it.
 [group('nix')]