Fix arbitrary code execution vulnerability

[lennartkoebe]

Internally unix-print uses exec calls which are very easy to exploit.

e.g.

const file = "assets/file.pdf";
const userProvidedPrinterName = "&& user_provided_evil_command";
print(file, userProvidedPrinterName);

This was fixed by using execFile instead.

see https://nodejs.org/api/child_process.html#child_processexecfilefile-args-options-callback