Download ICON grid without locking

[bouweandela]

Description

The file lock used in the ICON grid download function is causing issues with parallelization. This pull request removes the use of that lock.

See https://github.com/ESMValGroup/ESMValCore/issues/2341#issuecomment-1968744040.

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Checklist

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• [x] 🧪 The new functionality is relevant and scientifically sound
• [x] 🛠 This pull request has a descriptive title and labels
• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🛠 Unit tests have been added
• [x] 🛠 Changes are backward compatible
• [x] 🛠 The list of authors is up to date
• [x] 🛠 All checks below this pull request were successful

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.12%. Comparing base (2be264d) to head (cf5f9f9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2359 +/- ## ======================================= Coverage 94.11% 94.12% ======================================= Files 241 241 Lines 13381 13387 +6 ======================================= + Hits 12594 12600 +6 Misses 787 787 ```

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