nix: migrate to 'ruff' formatting

Having just the one tool is better.

Incidentally, this fixes the ternary formatting, which was messed up in
recent `black` versions.

2021/d25/ex1/ex1.py

@@ -63,7 +63,9 @@ def solve(input: List[str]) -> int:
                     (
                         "v"
                         if Point(x, y) in map.south
-                        else ">" if Point(x, y) in map.east else "."
+                        else ">"
+                        if Point(x, y) in map.east
+                        else "."
                     )
                     for y in range(map.dimensions.y)
                 )