Improve ci

[InCogNiTo124]

Closes #237

[InCogNiTo124]

I would also suggest:

• Bumping actions/checkout to v3
• Bumping ubuntu-18.04 to ubuntu-20.04

[codecov-commenter]

Codecov Report

Merging #240 (7f6409e) into master (c5675ac) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
+ Coverage   55.55%   55.60%   +0.04%     
==========================================
  Files          96       96              
  Lines        8953     8953              
==========================================
+ Hits         4974     4978       +4     
+ Misses       3979     3975       -4     

Impacted Files	Coverage Δ
...rithms/linfa-trees/src/decision_trees/algorithm.rs	49.65% <0.00%> (+1.38%)	⬆️

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[YuhanLiin]

Thanks for the PR!