ci: Add test-ubuntu-20.04 job

[peaceiris]

cf. #268

• Add test job for ubuntu 20.04
• Bumps mdBook from 0.3.6 to 0.3.7

[codecov]

Codecov Report

Merging #270 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #270   +/-   ##
=======================================
  Coverage   53.03%   53.03%           
=======================================
  Files           4        4           
  Lines         247      247           
  Branches       42       42           
=======================================
  Hits          131      131           
  Misses        116      116           

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