fix: avoid-border-all is triggered even when it is not a const

[vlkonoshenko]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[X] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

#837

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 424

• Total lines of source code: 7448

• Total classes: 290

• Average Cyclomatic Number per line of code: 0.35

• Average Source Lines of Code per method: 6

• Total tech debt: 1318.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #841 (2a67942) into master (f6758f9) will increase coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head 2a67942 differs from pull request most recent head 920a9b9. Consider uploading reports for the commit 920a9b9 to get more accurate results

@@            Coverage Diff             @@
##           master     #841      +/-   ##
==========================================
+ Coverage   87.25%   87.32%   +0.07%     
==========================================
  Files         279      279              
  Lines        5932     5934       +2     
==========================================
+ Hits         5176     5182       +6     
+ Misses        756      752       -4     

Impacted Files	Coverage Δ
...zer/rules/rules_list/avoid_border_all/visitor.dart	100.00% <100.00%> (ø)
...metrics/metrics_list/number_of_methods_metric.dart	100.00% <0.00%> (+17.39%)	⬆️

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