Update build.yml

[dorssel]

Use global.json for actions/setup-dotnet

[dorssel]

actions/setup-dotnet does not allow c-style comments in global.json, which doesn't play well with embedded REUSE license comment.
Reported at actions/setup-dotnet#254.

[codecov]

Codecov Report

Merging #144 (9f3d8c7) into master (7e21c5f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #144   +/-   ##
=======================================
  Coverage   42.37%   42.37%           
=======================================
  Files          30       30           
  Lines        2589     2589           
=======================================
  Hits         1097     1097           
  Misses       1492     1492           

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