Oh, I've never actually used the Plesk task scheduler before. I just assumed it worked the same way as cPanel did. That's actually really nice security on Plesk's part. Thanks for letting us know. Here's an alternate way to compile your C++ executable.
In httpdocs/cgi-bin/ create a file named .htaccess with 644 permissions with these contents
Options +ExecCGI
AddHandler cgi-script .cgi .pl .py .sh
Also in httpdocs/cgi-bin/ create a file named compile.sh with 755 permissions with these contents
#!/bin/bash
exec 2>&1
echo -e "Content-type: text/plain\n"
/usr/bin/gcc /home/z0n1n0z.helioho.st/httpdocs/cgi-bin/test.cpp -o /home/z0n1n0z.helioho.st/httpdocs/cgi-bin/test.cgi
Then execute the compile in your browser by going to z0n1n0z.helioho.st/cgi-bin/compile.sh
After the C++ executable is compiled I recommend changing the compile.sh script to non-executable, for instance 644 permissions, so that people can't run it over and over and cause high load on your account.
I haven't tested the above, but it should work. Let us know if you run into any issues.