CI: Ensure code coverage reporting will work in February 2022 and beyond

[fingolfin]

See gap-actions/process-coverage#10

[codecov]

Codecov Report

Merging #10 (489a626) into master (507caee) will decrease coverage by 4.39%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   46.91%   42.52%   -4.40%     
==========================================
  Files          41       41              
  Lines        1230     1230              
  Branches        0      200     +200     
==========================================
- Hits          577      523      -54     
  Misses        653      653              
- Partials        0       54      +54     

Impacted Files	Coverage Δ
src/4ti2groebner.cc	55.17% <0.00%> (-13.80%)	⬇️
src/4ti2zsolve.cc	38.20% <0.00%> (-11.24%)	⬇️
src/4ti2normalform.cc	27.69% <0.00%> (-10.77%)	⬇️
src/4ti2minimize.cc	35.59% <0.00%> (-8.48%)	⬇️
src/4ti2daco.cc	43.01% <0.00%> (-7.53%)	⬇️
src/basicxch.cc	65.00% <0.00%> (-5.00%)	⬇️

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