change to sites.ecmwf instead of nexus

[mathleur]

Description

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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.37%. Comparing base (1645e9a) to head (c32c9c5).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #412   +/-   ##
========================================
  Coverage    87.37%   87.37%           
========================================
  Files           42       42           
  Lines         5274     5274           
========================================
  Hits          4608     4608           
  Misses         666      666           

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