mkosi: explicitly disable KVM in GHA runs

mkosi detects whether /dev/kvm is available and uses it if it is. But
some GHA hosts have it, but it's broken and not supported, so we need
to explicitly disable it.

(cherry picked from commit c13e6c7)

Author: bluca

.github/workflows/mkosi.yml

@@ -100,6 +100,8 @@ jobs:
         [Host]
         ExtraSearchPaths=!*
         QemuVsock=yes
+        # Sometimes we run on a host with /dev/kvm, but it is broken, so explicitly disable it
+        QemuKvm=no
         Ephemeral=yes
         EOF