Fix Migration

[AndrewJGaut]

It now appears to work on Prod.

Fixes issue we were saying that gave the following stack trace:

Exception in thread Thread-140:
Traceback (most recent call last):
  File "/opt/conda/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/opt/conda/lib/python3.7/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/codalab-worksheets/codalab/lib/upload_manager.py", line 308, in create_index
    indexFilePath=tmp_index_file.name,
  File "/opt/codalab-worksheets/codalab/lib/beam/SQLiteIndexedTar.py", line 273, in __init__
    self._createIndex(self.tarFileObject)
  File "/opt/codalab-worksheets/codalab/lib/beam/SQLiteIndexedTar.py", line 619, in _createIndex
    for tarInfo in loadedTarFile:
  File "/opt/conda/lib/python3.7/tarfile.py", line 2403, in __iter__
    tarinfo = self.next()
  File "/opt/conda/lib/python3.7/tarfile.py", line 2279, in next
    self.fileobj.seek(self.offset - 1)
  File "/opt/conda/lib/python3.7/tarfile.py", line 517, in seek
    self.read(self.bufsize)
  File "/opt/conda/lib/python3.7/tarfile.py", line 537, in read
    buf = self._read(size)
  File "/opt/conda/lib/python3.7/tarfile.py", line 545, in _read
    return self.__read(size)
  File "/opt/conda/lib/python3.7/tarfile.py", line 568, in __read
    buf = self.fileobj.read(self.bufsize)
  File "indexed_gzip/indexed_gzip.pyx", line 797, in indexed_gzip.indexed_gzip._IndexedGzipFile.readinto
indexed_gzip.indexed_gzip.ZranError: zran_read returned error: ZRAN_READ_FAIL (file: n/a)

Looks like it was due to some issue with our tar_gzip code.

[epicfaace]

Let's fix format