Github Actions にcheckoutを追加（※Auto Merge）

[shinya]

以前、申請したPRである #827 において、
failed to run git: fatal: not a git repository (or any of the parent directories): .git
というエラーが発生。リポジトリのチェックアウトが必要なのにできていないというミスがあったので、追加で対応を行いました。

具体的には、承認処理が行われる前にチェックアウトのstepを追加しています。

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d68e826) 55.38% compared to head (4fa87c1) 55.38%.
Report is 6 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #835   +/-   ##
=======================================
  Coverage   55.38%   55.38%           
=======================================
  Files          75       75           
  Lines        8908     8908           
=======================================
  Hits         4934     4934           
  Misses       3974     3974           

Flag	Coverage Δ
tests	55.38% <ø> (ø)

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