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.

Changed files
+1 -1
nixos
modules
services
networking
+1 -1
nixos/modules/services/networking/mosquitto.nix
···
'' + 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);