FIX downloading volume from connector

[olethanh]

Since the pydantic upgrade the / is automatically added thus it resulted in a double // e.g http://localhost:4021//download/data/5f31b0706f59404fad3d0bff97ef89ddf24da4761608ea0646329362c662ba5 which made the vm-connector return 404

It was fixed for other download function from the vm connector but this one was missed

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 64.56%. Comparing base (51dc7b1) to head (d88bf33).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aleph/vm/storage.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #796   +/-   ##
=======================================
  Coverage   64.56%   64.56%           
=======================================
  Files          78       78           
  Lines        7092     7092           
  Branches      598      598           
=======================================
  Hits         4579     4579           
  Misses       2313     2313           
  Partials      200      200           

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