mkosi: update debian commit reference

* 07a294d0c6 Do not mask systemd-gpt-auto-generator in upstream CI builds
* 5636398bf7 Backport patch to fix test failures with tzdata 2024b-1
* 354ded4946 Update changelog for 256.7-2 release
* e38c7c5345 Backport fixes for upstream autopkgtest suite
* 249676834c Disable utmp support, not y2038 safe
* 822d44da42 initramfs-tools: support missing /etc/udev/udev.conf
* ad71ebf700 systemd-boot: depend on systemd for kernel-install
* 5bf7008ef8 d/systemd.postinst: do not restart systemd-binfmt.service if masked
* 58d5aa1b41 d/rules: mask systemd-gpt-auto-generator on Ubuntu
* 481987d85c Update changelog for 256.7-1 release
* ce7f3d4b43 Revert "autopkgtest: skip TEST-64-UDEV-STORAGE due to qemu crash"
* 7007e73b22 Mark dependencies on clang and bpftool as :native
*   0e120cf704 Update upstream source from tag 'upstream/256.7'
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| * 914aae055c New upstream version 256.7
* fcea89cb00 d/t/upstream: honor /etc/apt configured by autopkgtest