chore(deps): update nixpkgs pin

Nixpkgs has removed support for x86_64-darwin, so removing that from
CI's flake check.

A x86_64-darwin binary is still built via Stack.

test/io/config.py

@@ -4,7 +4,6 @@ import shutil
 import uuid
 import yaml
 
-
 BASEDIR = pathlib.Path(os.path.realpath(__file__)).parent
 CONFIGSDIR = BASEDIR / "configs"
 FIXTURES = yaml.load(