mkosi: update debian commit reference

* aa17b7ddf9 Fix stage1 build
* 2c13391e33 Update changelog for 256.5-1 release
* 7d13196926 autopkgtest: skip TEST-64-UDEV-STORAGE due to qemu crash
* 47769e8d7c Drop patch merged upstream
*   4e8e9315b5 Update upstream source from tag 'upstream/256.5'
|\
| * 71b885347d New upstream version 256.5
* 89a33e5408 d/e/checkout-upstream: undo quilt patches before switching debian branch
* 3c942ecb0d d/e/checkout-upstream: do not rebase on main when building stable branches

mkosi.images/build/mkosi.conf.d/debian-ubuntu/mkosi.conf

@@ -9,7 +9,7 @@ Environment=
         GIT_URL=https://salsa.debian.org/systemd-team/systemd.git
         GIT_SUBDIR=debian
         GIT_BRANCH=debian/master
-        GIT_COMMIT=6e0f4f74bad23b54bfbca201cba4d3b8323fbbd6
+        GIT_COMMIT=aa17b7ddf9633b4b0d06fdad3281137ba2851721
         PKG_SUBDIR=debian
 
 Packages=