mkosi: Set mitigations=off

Might as well disable mitigations for these VMs. Who knows, maybe it'll
speed something up.

(cherry picked from commit 36645ec255)
(cherry picked from commit 9f549793ee)

mkosi.conf

@@ -79,6 +79,7 @@ KernelCommandLine=
         systemd.mask=isc-dhcp-server.service
         systemd.mask=mdmonitor.service
         psi=1
+        mitigations=off
 
 KernelModulesInitrdExclude=.*
 KernelModulesInitrdInclude=default