Private key on Ubuntu is created as world-readable file

[smarnach]

Describe the bug
The private key of the hbbs server is created as world-readable file.

Describe the environment

Debian package on Ubuntu 20.04, version 1.1.14

How to Reproduce the bug
Steps to reproduce the behavior:

1. Install the package.
2. The file /var/lib/rustdesk-server/id_ed25519 is now readable by any user on the system, i.e. it has mode 0644.

Expected behavior
The private key should only be readable by the user the servers are running as, i.e. it should have mode 0600 and owned by the correct user.