Avoid bashisms in the call to acme.sh
This command is using features that are not POSIX-compliant (the [[ and the || in them), that despite being supported by many shells (including bash) are not available in dash.
Since system() runs the code with /bin/sh that is an interface defined as only being POSIX-compliant, with no extensions, it should not presume a specific shell in that.
Closes #6326 (closed)