mkosi: Only keep failed test journals and other logs for 7 days

The default retention of 90 days seems a bit long, so drop it to 7
days.

Author: DaanDeMeyer

.github/workflows/mkosi.yml

@@ -194,6 +194,7 @@ jobs:
         path: |
           build/test/journal/*.journal
           build/meson-logs/*
+        retention-days: 7
 
     - name: Archive packages
       uses: actions/upload-artifact@v4