Multiple users with hashedPassword is broken in mosquitto

If you have more than 1 User with hasedPassword Option set it generates 

```
rm -f /var/lib/mosquitto/passwd
touch /var/lib/mosquitto/passwd
echo 'user1:$6$xxx' > /var/lib/mosquitto/passwd
echo 'user2:$6$xxx' > /var/lib/mosquitto/passwd
```

Which ends up in only having 1 user.

nixos/modules/services/networking/mosquitto.nix

@@ -212,7 +212,7 @@ in
       '' + concatStringsSep "\n" (
         mapAttrsToList (n: c:
           if c.hashedPassword != null then
-            "echo '${n}:${c.hashedPassword}' > ${cfg.dataDir}/passwd"
+            "echo '${n}:${c.hashedPassword}' >> ${cfg.dataDir}/passwd"
           else optionalString (c.password != null)
             "${pkgs.mosquitto}/bin/mosquitto_passwd -b ${cfg.dataDir}/passwd ${n} ${c.password}"
         ) cfg.users);