unix: Don't create world-writable files when safe save is enabled

When the "filesystem/on_save/safe_save_on_backup_then_rename" option is
enabled files are created with 0666 permissions (-rw-rw-rw-) which is
too loose. Use 0644 (-rw-r--r--) instead which is how the files would
normally be created with the setting disabled and the system umask taken
into account.

drivers/unix/file_access_unix.cpp

@@ -97,7 +97,7 @@ Error FileAccessUnix::open_internal(const String &p_path, int p_mode_flags) {
 			last_error = ERR_FILE_CANT_OPEN;
 			return last_error;
 		}
-		fchmod(fd, 0666);
+		fchmod(fd, 0644);
 		path = String::utf8(cs.ptr());
 
 		f = fdopen(fd, mode_string);