dependabot: Add zizmor to lint dependencies

This is for better dependabot grouping

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

.github/dependabot.yml

@@ -19,6 +19,7 @@ updates:
         - "mypy"
         - "ruff"
         - "tox"
+        - "zizmor"
     dependencies:
       # Python (developer) runtime dependencies. Also any new dependencies not
       # caught by earlier groups