Use sops for private key distribution

nixos/modules/borgbackup.nix

@@ -131,6 +131,14 @@ in {
         readWritePaths = ["/home/fsaccount"];
       };
     };
+    environment.persistence.${config.impermanence.name} = {
+      users.fsaccount.files = [
+        {
+          file = ".ssh/known_hosts";
+          parentDirectory = {mode = "u=rwx,g=,o=";};
+        }
+      ];
+    };
     sops.secrets.backupKey = {
       sopsFile = ../machines/bragi/backupKey.yaml;
       path = "/home/fsaccount/.ssh/backupKey";