Make migration deterministic over filesystem

[berland]

The test of this function has exposed flakyness, which can be reproduced by tweaking the order of files returned by glob().

pathlib.Path.glob() returns filenames in arbitrary order.

Issue
Resolves #12863

Approach
🧠

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.63%. Comparing base (673fdcb) to head (c8c240b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12865      +/-   ##
==========================================
- Coverage   90.64%   90.63%   -0.01%     
==========================================
  Files         440      440              
  Lines       30460    30460              
==========================================
- Hits        27610    27607       -3     
- Misses       2850     2853       +3     

Flag	Coverage Δ
cli-tests	37.21% <0.00%> (ø)
gui-tests	67.88% <0.00%> (ø)
performance-and-unit-tests	77.05% <100.00%> (-0.01%)	⬇️
test	37.35% <0.00%> (ø)

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[scout-team-app]

Successfully created backport PR for version-20.0:

• [Backport version-20.0] Make migration deterministic over filesystem #12866

[codspeed-hq]

Merging this PR will not alter performance

✅ 34 untouched benchmarks

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_{Comparing berland:fix_flaky_glob (c8c240b) with main (673fdcb)}