Use new remote test data address

[sandorkertesz]

Description

Changes remote test data site to sites.ecmwf.int from get.ecmwf.int.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.70588% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.66%. Comparing base (fe3bfda) to head (7130d71).
⚠️ Report is 258 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/netcdf/test_netcdf_geography.py	16.66%	5 Missing ⚠️
tests/xr_engine/test_xr_torch.py	0.00%	3 Missing ⚠️
tests/readers/test_netcdf_reader.py	0.00%	2 Missing ⚠️
tests/xr_engine/test_xr_write.py	75.00%	2 Missing ⚠️
tests/xr_engine/test_xr_chunks.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #775   +/-   ##
========================================
  Coverage    85.65%   85.66%           
========================================
  Files          172      172           
  Lines        13171    13179    +8     
  Branches       637      637           
========================================
+ Hits         11282    11290    +8     
  Misses        1698     1698           
  Partials       191      191           

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