ci: Add ubuntu-20.04 to matrix

[peaceiris]

cf. Add Ubuntu 20.04 focal · Issue #228 · actions/virtual-environments

[codecov]

Codecov Report

Merging #336 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #336   +/-   ##
=======================================
  Coverage   57.60%   57.60%           
=======================================
  Files           4        4           
  Lines         250      250           
  Branches       43       43           
=======================================
  Hits          144      144           
  Misses        106      106           

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