fix bug in common/io.py when retrieving the fstep in ZarrIO

[iluise]

Description

There is a bug in common/io.py when retrieving the fstep info from ZarrIO.
the code looks for the 0/ERA5_in/0 group even if this stream was used just in input.
Changed it to loop over the streams and actually open only the first available stream in output.

Issue Number

Closes #2770

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Checklist before asking for review

• I have performed a self-review of my code
• My changes comply with basic sanity checks:
  • I have fixed formatting issues with ./scripts/actions.sh lint
  • I have run unit tests with ./scripts/actions.sh unit-test
  • I have documented my code and I have updated the docstrings.
  • I have added unit tests, if relevant
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FastEvaluation

• I have updated the public documentation if necessary

[clessig]

@grassesi : could you have a look?