Update GithubUpdater.php

Temp fix to fix error that may occur if the PHP installation on the server doesn't have access to the system's temporary directory or the necessary permissions to create files in it.

includes/updater/GithubUpdater.php

@@ -209,7 +209,8 @@ public function modify_transient( $transient ) {
 	 */
 	private function get_tmpfile_data( $string ) {
 
-		$temp = tmpfile();
+		$temp_file = wp_tempnam();
+		$temp = fopen($temp_file, 'r+');
 		$tmpfilename = stream_get_meta_data($temp)['uri'];
 		fwrite( $temp, $string);