Report nix-eval as failed if effect fails to run

Signed-off-by: magic_rb <richard@brezak.sk>

Author: MagicRB

buildbot_nix/buildbot_nix/__init__.py

@@ -1109,6 +1109,7 @@ def nix_eval_config(
                 "list",
                 # fmt: on
             ],
+            flunkOnFailure=True,
             # TODO: support other branches?
             doStepIf=lambda c: c.build.getProperty("branch", "")
             == project.default_branch,