Fix local file creation when checking for an already synced file on the HDD

Error

Sentry Issue: NUXEO-DRIVE-2AP

IsADirectoryError: [Errno 21] Is a directory: '/home/USER/Nuxeo Drive'
  File "nxdrive/engine/processor.py", line 270, in _execute
  File "nxdrive/engine/processor.py", line 1053, in _synchronize_remotely_modified
  File "nxdrive/engine/processor.py", line 1012, in _update_remotely
  File "nxdrive/engine/processor.py", line 985, in _download_content
  File "shutil.py", line 248, in copy
  File "shutil.py", line 120, in copyfile

Analysis

When downloading a file, we first check for a "duplicate" one on the HDD, e.g. a file synced, in another location, with the same digest of the one we want to download. This is an interesting improvement to prevent useless multiple downloads of the same binary file.

The line is https://github.com/nuxeo/nuxeo-drive/blob/release-4.2.0/nxdrive/engine/processor.py#L978.
It is calling get_valid_duplicate_file() that checks for the remote digest, as seen here: https://github.com/nuxeo/nuxeo-drive/blob/release-4.2.0/nxdrive/engine/dao/sqlite.py#L1137.

In the Sentry event, the found doc pair seems in a weird state, its local_path is set to /, and so it is converted to the local sync folder root.

Fix

Instead of just checking for the same remote digest, we should check for the same local digest too to better ensure the file is synced.