remove accept-flake-config from n-e-j invocation

it's off by default and thus not representative of user flake setup, we
don't use it anyway, and it's a security risk to boot. there is no good
reason to enable this in any setting that is not perfectly trusted, and
even there it is not such a great idea due to the impurity it requires.

buildbot_nix/__init__.py

@@ -461,9 +461,6 @@ def nix_eval_config(
                 str(worker_count),
                 "--max-memory-size",
                 str(max_memory_size),
-                "--option",
-                "accept-flake-config",
-                "true",
                 "--gc-roots-dir",
                 drv_gcroots_dir,
                 "--force-recurse",