fix race condition by defaulting to local mount prior to R2 fetch

[lianakoleva]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fix a race condition where locally-accessible Lightning Storage input files were fetched from R2 but did not exist there yet. Now, Lightning Storage will be used when locally accessible.

PR review

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81%. Comparing base (808fe07) to head (24cbf28).
⚠️ Report is 1 commits behind head on main.
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Additional details and impacted files

@@         Coverage Diff         @@
##           main   #821   +/-   ##
===================================
  Coverage    81%    81%           
===================================
  Files        54     54           
  Lines      7617   7617           
===================================
+ Hits       6144   6146    +2     
+ Misses     1473   1471    -2     

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