nixos/networking: add hostname to /etc/hosts by default

We use `127.0.1.1` instead of `127.0.0.1` because some applications will fail if
`127.0.0.1` resolves to something other than `localhost`.

Debian does the same.

See #1248 and #36261.

nixos/modules/config/networking.nix

@@ -215,6 +215,8 @@ in
 
     networking.hosts = {
       "127.0.0.1" = [ "localhost" ];
+    } // optionalAttrs (cfg.hostName != "") {
+      "127.0.1.1" = [ cfg.hostName ];
     } // optionalAttrs cfg.enableIPv6 {
       "::1" = [ "localhost" ];
     };