Fix zsh impermanence

https://logs.nix.samueldr.com/nixos-chat/2020-06-07
Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

hosts/common/configs/zsh.nix

@@ -1,3 +1,6 @@
 {
-  programs.zsh.enable = true;
+  programs.zsh = {
+    enable = true;
+    histFile = "/var/lib/zsh/history";
+  };
 }

hosts/common/default.nix

@@ -34,13 +34,13 @@
         "/etc/nixos"
         "/etc/NetworkManager/system-connections"
         "/var/lib/nixos"
-        "/var/log"
         "/var/lib/systemd/coredump"
+        "/var/lib/zsh"
+        "/var/log"
       ];
       files = [
         "/etc/ssh/ssh_host_ed25519_key"
         "/etc/machine-id"
-        "/root/.zsh_history"
       ];
     };