CI: Update codecov upload

[chfast]

Use CircleCI orb for codecov, use _ in flag names because of the bug in the script (codecov/codecov-circleci-orb#175).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.31%. Comparing base (3417de7) to head (9d5ecbf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #910      +/-   ##
==========================================
+ Coverage   98.29%   98.31%   +0.02%     
==========================================
  Files         131      131              
  Lines       15968    14979     -989     
==========================================
- Hits        15696    14727     -969     
+ Misses        272      252      -20     

Flag	Coverage Δ
ethereum-tests	?
ethereum-tests-silkpre	?
ethereum_tests	28.07% <ø> (?)
ethereum_tests_silkpre	19.69% <ø> (?)
execution-spec-tests	?
execution_spec_tests	19.09% <ø> (?)
unittests	94.03% <ø> (-0.22%)	⬇️

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

see 105 files with indirect coverage changes