Use lcov format when sending coverage results

[Mingun]

I confused https://codecov.io and https://coveralls.io when I was preparing coverage and chose the coverage+ format, although it is not required in this case and the grcov Readme.md recommends using lcov to upload to codecov.io.

[codecov-commenter]

Codecov Report

Merging #392 (dc180b1) into master (015079b) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #392      +/-   ##
==========================================
- Coverage   58.80%   58.76%   -0.04%     
==========================================
  Files          19       19              
  Lines        9537     9538       +1     
==========================================
- Hits         5608     5605       -3     
- Misses       3929     3933       +4     

Flag	Coverage Δ
unittests	58.76% <ø> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/de/map.rs	95.60% <0.00%> (-1.10%)	⬇️
src/de/mod.rs	61.51% <0.00%> (-0.12%)	⬇️
src/reader.rs	87.96% <0.00%> (-0.09%)	⬇️
src/lib.rs	25.78% <0.00%> (-0.02%)	⬇️

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