mkosi: remove ?priority(required) from debian pkg list

Requires apt 2.3.10 which is not available in stable, and also doesn't
seem necessary, same packages are installed anyway
Add tzdata manually, as that doesn't seem to be pulled in via the CI.

mkosi.conf.d/20-debian.conf

@@ -8,7 +8,6 @@ Release=testing
 
 [Content]
 Packages=
-        ?priority(required)
         btrfs-progs
         cryptsetup-bin
         dbus-broker
@@ -48,6 +47,7 @@ Packages=
         python3-pytest
         quota
         systemd-sysv
+        tzdata
         xxd
 
 BuildPackages=