test: skip ERA5 tests when CDS API is not configured

[FabianHofmann]

Changes proposed in this Pull Request

Fork PRs (e.g. #483) fail on macOS and Windows CI because:

• The daily scheduled run only populates the cutout cache on Ubuntu. macOS/Windows rely on cross-OS cache restoration (enableCrossOsArchive: true), which seems to be flaky.
• When the cache miss occurs, the test fixtures in conftest.py call cutout.prepare(), which tries to download ERA5 data from the CDS API.
• GitHub Actions does not expose repository secrets to fork PRs for security, so CDSAPI_URL is empty, causing a MissingSchema error.

This PR introduces a _prepare_era5_cutout helper in test/conftest.py that checks whether the cached cutout file exists before calling cutout.prepare(). If the file is missing and CDSAPI_URL is not set, the test is skipped with a clear message instead of erroring out. This makes fork PR CI runs robust regardless of cache availability.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.