Fix occasional sops-nix error

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

hosts/common/configs/system/sops/default.nix

@@ -1,4 +1,9 @@
-{ inputs, pkgs, ... }:
+{
+  config,
+  inputs,
+  pkgs,
+  ...
+}:
 {
   imports = [ inputs.sops-nix.nixosModules.sops ];
 
@@ -9,7 +14,7 @@
 
   sops.age = {
     generateKey = true;
-    sshKeyPaths = [ "/persist/etc/ssh/ssh_host_ed25519_key" ];
+    sshKeyPaths = [ config.environment.persistence."/persist"."/etc/ssh/ssh_host_ed25519_key".source ];
     keyFile = "/var/lib/sops-nix/key.txt";
   };
 }