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


Signed-off-by: Mattia Rizzolo <mattia@mapreri.org>

server/lib/classes/letsencrypt.inc.php

@@ -74,7 +74,7 @@ class letsencrypt {
 			$cert_arg = '--fullchain-file ' . escapeshellarg($bundle_file) . ' --cert-file ' . escapeshellarg($cert_file);
 		}
 
-		$cmd = 'R=0 ; C=0 ; ' . $letsencrypt . ' --issue ' . $cmd . ' -w /usr/local/ispconfig/interface/acme --always-force-new-domain-key --keylength 4096; R=$? ; if [[ $R -eq 0 || $R -eq 2 ]] ; then ' . $letsencrypt . ' --install-cert ' . $cmd . ' --key-file ' . escapeshellarg($key_file) . ' ' . $cert_arg . ' --reloadcmd ' . escapeshellarg($this->get_reload_command()) . ' --log ' . escapeshellarg($conf['ispconfig_log_dir'].'/acme.log') . '; C=$? ; fi ; if [[ $C -eq 0 ]] ; then exit $R ; else exit $C  ; fi';
+		$cmd = 'R=0 ; C=0 ; ' . $letsencrypt . ' --issue ' . $cmd . ' -w /usr/local/ispconfig/interface/acme --always-force-new-domain-key --keylength 4096; R=$? ; if [ $R -eq 0 -o $R -eq 2 ] ; then ' . $letsencrypt . ' --install-cert ' . $cmd . ' --key-file ' . escapeshellarg($key_file) . ' ' . $cert_arg . ' --reloadcmd ' . escapeshellarg($this->get_reload_command()) . ' --log ' . escapeshellarg($conf['ispconfig_log_dir'].'/acme.log') . '; C=$? ; fi ; if [ $C -eq 0 ] ; then exit $R ; else exit $C  ; fi';
 
 		return $cmd;
 	}