nixos/security: add acme through dns challenge

few side refactors of this:
- no more `dns.domain`, it all must rely on `fqdn`, prevents
  inconsistencies.
- also added an specific host `tuxcord-acmetest` that uses the key zone
  for `nix.tuxcord.net` to test certificate pulling.

nixos/hosts/tuxcord-acmetest/default.nix

@@ -0,0 +1,12 @@
+{ config, ... }:
+let
+  inherit (config.networking) fqdn;
+in
+{
+  acme.enable = true;
+  acme.rfc2136.nameserver = "tuxcord.net";
+
+  dns.enable = true;
+  networking.fqdn = "nix.tuxcord.net";
+  time.timeZone = "Europe/Madrid";
+}