mkosi: update debian commit reference

mkosi: update debian commit reference

* 48fabbd5d2 Install new sd-keyutil binary in sd-repart package
* 6dd9ab10fe Update changelog for 257~rc1-4 release
* 6dd325f04b Backport patch to fix TEST-07-PID1 integration test
* 5988cc60ee Update changelog for 257~rc1-3 release
* cf3a2f7ccc Backport another patch to fix test failure on buildd
* 5d6a226dbb Update changelog for 257~rc1-2 release
* ebe97c52c8 Backport patch to fix unit test failure on buildd
* 21f63b20bb Update changelog for 257~rc1-1 release
* 0dfec51bbb d/copyright: remove pattern for directory that is no longer present
* 337b3bb2dd Ignore Lintian warning dh-exec-script-without-dh-exec-features
* b680e6b448 List new libsystemd0 symbols
* 3c00aa000c gbp.conf: use --first-parent for dch to avoid upstream commits
* d53ecc7769 Install new files
* 546e8c9137 Drop all patches, merged upstream
* 6757597480 Update upstream source from tag 'upstream/257_rc1'
* 4b82805020 gbp.conf: switch upstream branch to full upstream history
* e60c637a95 gbp.conf: enable signing tags by default
* 2ad27b63c4 Update changelog for 256.7-3 release
* a212c36c54 systemd-boot: provide integration with shim