mkosi: update debian commit reference to 6f4d90be5cb4075954f0a36653105e586a9a1fa9

mkosi: update debian commit reference to 6f4d90be5cb4075954f0a36653105e586a9a1fa9

* 6f4d90be5c Do not install autovt@ for upstream builds
* 8cc28a6b82 Install new files for upstream build
* 0d15255073 Use deb-systemd-invoke to reexec instead of manual calls
* db04e5fa0b Use dh_installsystemd to handle journald and networkd
* d8756a4c82 Use dh_installsystemd more to manage units
* 40b23b0d5d d/tests: drop tests-in-lxd
* 5821c5a350 d/control: have systemd-boot depend on efibootmgr for amd64 and arm64 only

Follow-up for 072e72424b2e6da1c96489ef6996f49fabd46474