mkosi: update debian commit reference

* e8b7c9a4dd Install 81-net-bridge.rules
* 50d2997a07 Install systemd-creds bash completion
* ff0c42823c test: fix flaky boot-and-services test
* 2a19dee4ba test: fix flaky boot-and-services test
* a15a0bfe60 Update changelog for 257-2 release
* c24eafcb7e Backport patches to fix test failures
* 29840f9b68 udev: install dmi_memory_id and its rules on riscv64
* 44893bdb32 Update changelog for 257-1 release
* 7f71d995fb Update symbols file for v257
* 2dd2b80499 Update upstream source from tag 'upstream/257'
* 51a3271a85 Update changelog for 257~rc3-1 release
* 8e687227c5 Update symbols for 257~rc3
* c9bae527d6 Drop patches, merged upstream
* e8cf329870 Update upstream source from tag 'upstream/257_rc3'
* 794457516d autopkgtest: fix one more tzdata dependency
* 16bb143da1 Bump version in tzdata dependency due to p-u upload
* f2ddf70604 sysctl: Add file trigger on /usr/lib/sysctl.d to restart systemd-sysctl
* 79260cb0f4 Increase minimum sections in stub PE header on arm64/armhf/riscv64 to 500
* ed3af24635 systemd-ukfy: recommend systemd-boot-efi for the stub

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=51cd22f3684725a1b199012555e7378f2f468c16
+        GIT_COMMIT=e8b7c9a4dda2888fec41ac020e1194cfdf57eb5d
         PKG_SUBDIR=debian
 
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