Add Write Permission to File to Allow truncate() Operation to Succeed

[edverma]

I got the following error and am proposing this change to remediate.

Status code: 1
Error: Traceback (most recent call last):
File "/usr/src/app/preload.py", line 978, in
result = method(**data.get("parameters", {}))
File "/usr/src/app/preload.py", line 923, in main_preload
preload(additional_bytes, app_data, splash_image_path)
File "/usr/src/app/preload.py", line 713, in preload
part.resize(additional_bytes)
File "/usr/src/app/preload.py", line 434, in resize
self._resize_last_partition_of_disk_image(additional_bytes)
File "/usr/src/app/preload.py", line 334, in _resize_last_partition_of_disk_image
expand_file(self.image, additional_bytes)
File "/usr/src/app/preload.py", line 523, in expand_file
f.truncate(size + additional_bytes)
PermissionError: [Errno 13] Permission denied