enable auto-deletion of PR caches

[wheeheee]

https://github.com/julia-actions/cache?tab=readme-ov-file#cache-retention

[codecov[bot]]

Codecov Report

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

Comparison is base (be1a084) 97.56% compared to head (1a97b16) 97.56%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #537 +/- ## ======================================= Coverage 97.56% 97.56% ======================================= Files 18 18 Lines 3124 3124 ======================================= Hits 3048 3048 Misses 76 76 ```

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