mkosi: update debian commit reference

mkosi: update debian commit reference

* c7138e0b87 Configure default DNS servers for upstream CI builds
* bc5d1afe1e Drop out-of-tree localed patch and use D-Bus policy instead
* b5f8ababde autopkgtest: set Release= in mkosi.local.conf to distinguish testing vs unstable
* 323afafd80 autopkgtest: add allow-stderr to timedated test
* 0291f361e3 Install valrinkctl zsh completion file
* f40b9eba02 d/t/control: add Depends: lib{systemd,udev}-dev for upstream
* 3def595de3 d/t/upstream: ensure correct ubuntu codename is used
* 531bb6817e d/t/boot-and-services: fix a couple python sytax warnings
* 963ac13b7d d/t/boot-and-services: skip test_tmp_cleanup if tmp.mount is overridden