ci: pin actions/checkout to v2

[peaceiris]

No description provided.

[codecov]

Codecov Report

Merging #541 (f462dfb) into main (3ba244e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #541   +/-   ##
=======================================
  Coverage   61.17%   61.17%           
=======================================
  Files           4        4           
  Lines         273      273           
  Branches       48       48           
=======================================
  Hits          167      167           
  Misses        105      105           
  Partials        1        1           

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