github: make dependabot not update mkosi reference

The refernce to mkosi is manually updated by using tools/fetch-mkosi.py,
and should not be done automatically, as it may have a bigger impact.

Follow-up for c69c5dd58a.

.github/dependabot.yml

@@ -9,7 +9,9 @@ updates:
     groups:
       actions:
         patterns:
-          - "*"      
+          - "*"
+        exclude-patterns:
+          - "mkosi"
     open-pull-requests-limit: 2
   - package-ecosystem: "pip"
     directory: "/.github/workflows"