fix: fix file descriptor on corrupted WAL copy

[cheb0]

Description

Messed up flags while opening a file to copy a corrupted WAL.

---

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🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
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Name	Previous	Current	Ratio	Verdict
Indexer-4	42fb1c	7d43e1
	572784732.00 B/op	787873528.00 B/op	1.38	🔴

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.46%. Comparing base (42fb1c0) to head (55cd5b9).

Files with missing lines	Patch %	Lines
util/fs.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #465      +/-   ##
==========================================
- Coverage   70.47%   70.46%   -0.01%     
==========================================
  Files         232      232              
  Lines       18364    18364              
==========================================
- Hits        12942    12941       -1     
- Misses       4448     4449       +1     
  Partials      974      974              

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