pkg/debian: Add comment about compression

.github/workflows/debian.yml

@@ -93,6 +93,8 @@ jobs:
 
     - name: Build kernel
       env:
+        # The DPKG in Ubuntu 22.04 defaults to using ZSTD,
+        # which is not yet supported by the DPKG in Debian 11
         KDEB_COMPRESS: xz
       run: |
         cd pkg/debian/kernel/linux