Let's Encrypt not set as default CA when acme.sh is installed from the Let's Encrypt class

If a system has no Let's Encrypt client, the Let's Encrypt class installs acme.sh. But we missed adding a call to set Let's Encrypt as default CA when this became required for acme.sh. Only on install or update of ISPConfig, this is corrected.

We should set the default CA when installing acme.sh from the class.