Fix read and write racing on multiple processes.

[frostming]

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This PR address an issue that read/write on the update log can cause competition on multiple processes.
Close #1938

[codecov]

Codecov Report

Merging #1941 into master will decrease coverage by 0.09%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master    #1941      +/-   ##
==========================================
- Coverage   93.98%   93.89%   -0.10%     
==========================================
  Files          86       86              
  Lines        4225     4228       +3     
==========================================
- Hits         3971     3970       -1     
- Misses        254      258       +4     

Flag	Coverage Δ
#tests	93.89% <20.00%> (-0.10%)	⬇️

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Impacted Files	Coverage Δ
src/virtualenv/app_data/via_disk_folder.py	82.30% <0.00%> (-2.25%)	⬇️
src/virtualenv/util/path/_sync.py	93.10% <100.00%> (ø)
src/virtualenv/seed/embed/base_embed.py	96.22% <0.00%> (-1.89%)	⬇️

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[gaborbernat]

I'd expect some type of test here too 🤔