Add --diff as a default argument for isort (frequenz-floss#55)

Without it, isort just complains that imports are incorrectly formatted,
without showing how to fix it.

Author: shsms

src/frequenz/repo/config/nox/default.py

@@ -37,6 +37,7 @@
         "-v2",  # for verbose error messages.
     ],
     isort=[
+        "--diff",
         "--check",
     ],
     mypy=[