non-critical-infra/modules/mailserver: fix sops key format

Author: Mic92

non-critical-infra/modules/mailserver/default.nix

@@ -32,6 +32,7 @@
     # Next, look at `mail.txt` and update DNS accordingly.
     sopsFile = ../../secrets/nixos.org.mail.key.umbriel;
     path = "/var/dkim/nixos.org.mail.key";
+    format = "binary";
   };
 
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