azure blobstorage: Reset blob's content-encoding to allow check

This ensures the deposit checks in the elastic infra (using azure objstorage) is
working. There is currently no way to prevent the content-encoding from being detected
and stored in the blobstorage (various tryouts have been tested to no avail). That
content-encoding, is then preventing the checking/reading to work (failing [2]).

Following the documentation, a workaround has been implemented to reset a posteriori the
content-encoding [1].

It's not proper but, from repl experimentation, it's working which is better than the
actual failure.

[1] https://learn.microsoft.com/en-us/azure/storage/blobs/storage-blob-properties-metadata-python

[2]
```
HttpResponseError: ('Received response with content-encoding: gzip, but failed to decode it.', error('Error -3 while decompressing data: incorrect header check'))
```

Refs. swh/infra/sysadm-environment#5129