ci(os): use Python 3.13 for mkosi

Author: kvinwang

.github/workflows/mkosi-build.yml

@@ -54,6 +54,11 @@ jobs:
           ref: ${{ env.BASELINE_REVISION }}
           fetch-depth: 0
 
+      - name: Set up Python for mkosi
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.13'
+
       - name: Install host image assembly tools
         run: |
           sudo apt-get update