irmin-pack: only unlink results file if it exists

[metanivek]

Previously, this would log a warning for every new GC if the previous results file did not exist, which is the expected state when a GC begins.

Noticed this in logs when debugging an issue with Victor:

[WARNING] Unlinking temporary files from previous failed gc. Failed with error No such file or directory

[codecov-commenter]

Codecov Report

Merging #2240 (92d82a0) into main (4c38a4b) will decrease coverage by 0.03%. The diff coverage is 0.00%.

:exclamation: Current head 92d82a0 differs from pull request most recent head c01f9f1. Consider uploading reports for the commit c01f9f1 to get more accurate results

:mega: This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##             main    #2240      +/-   ##
==========================================
- Coverage   67.93%   67.91%   -0.03%     
==========================================
  Files         136      136              
  Lines       16603    16605       +2     
==========================================
- Hits        11279    11277       -2     
- Misses       5324     5328       +4     

Impacted Files	Coverage Δ
src/irmin-pack/unix/gc.ml	69.79% <0.00%> (-3.62%)	:arrow_down:

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more