Adjust the test data path for Nexus migration

[marcosbento]

Description

Adjust the test data path for Nexus migration. Instead of using https://get.ecmwf.int/repository/test-data/*project*/, the project will now access the data at https://sites.ecmwf.int/repository/*project*/test-data/.

This change also implies the use of the latest ecbuild, with the necessary adjustments to ecbuild_get_test_data/multidata functions.

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.