check home directory is writable or not

if home directory is not writable, libreoffice can not write tmp file so it can not complete convert

src/OfficeConverter/OfficeConverter.php

@@ -224,7 +224,12 @@ private function getAllowedConverter($extension = null)
      */
     private function exec($cmd, $input = '')
     {
-        $cmd = "export HOME=/tmp && " . $cmd;
+        // Cannot use $_SERVER superglobal since that's empty during UnitUnishTestCase
+        // getenv('HOME') isn't set on Windows and generates a Notice.
+        $home = getenv('HOME');
+        if (!is_writable($home)) {
+            $cmd = 'export HOME=/tmp && ' . $cmd;
+        }
         $process = proc_open($cmd, [0 => ['pipe', 'r'], 1 => ['pipe', 'w'], 2 => ['pipe', 'w']], $pipes);
         fwrite($pipes[0], $input);
         fclose($pipes[0]);