Fix Celery tests after conf-backcompat merge

[jscheffl]

It seems after merging of #64292 the pytests of Celery are failing because default provider config is not loaded anymore. This PR fixes the pytest as default config missing.

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[Nataneljpwd]

Tests seem to fail due to failed uv install (cache lock timeout), maybe it is a good idea to use no cache when building both with and without constraints at the same time?
It happens specifically on the iceberg-core package, I will investigate further
Or maybe increase the lock timeout of uv
What do you think?