nixos/impermanence: persist individual /etc/ssh files

sshd fails to start on first boot otherwise.

nixos/impermanence.nix

@@ -62,7 +62,6 @@
     hideMounts = true;
 
     directories = [
-      "/etc/ssh"
       "/export"
       "/mnt"
       "/var/db/sudo/lectured"
@@ -73,6 +72,10 @@
     ];
     files = [
       "/etc/machine-id"
+      "/etc/ssh/ssh_host_ed25519_key"
+      "/etc/ssh/ssh_host_ed25519_key.pub"
+      "/etc/ssh/ssh_host_rsa_key"
+      "/etc/ssh/ssh_host_rsa_key.pub"
     ];
   };