test: install bsdextrautils in debian minimal img

'rev' moved to bsdextrautils and TEST-50-DISSECT uses it, so it now
fails:

[   83.534905] bash[3581]: +++ grep '^now' /proc/timer_list
[   83.535479] bash[3582]: +++ cut '-d ' -f3
[   83.535774] bash[3583]: +++ rev
[   83.535774] bash[3583]: bash: line 1: rev: command not found

mkosi/mkosi.images/minimal-base/mkosi.conf.d/debian-ubuntu.conf

@@ -7,6 +7,7 @@ Distribution=|ubuntu
 [Content]
 PrepareScripts=%D/mkosi/mkosi.conf.d/debian-ubuntu/systemd.prepare
 Packages=
+        bsdextrautils
         hostname
         iproute2
         mount