Fix test summary reliability

[martincostello]

Avoid write conflicts when adding the coverage to the GITHUB_STEP_SUMMARY.

Bonus improvement of readability too.

Cherry-picked from #2531 where the addition of the net10.0 made it too likely to always happen.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.23%. Comparing base (284acee) to head (6d3d4e4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2543   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files         311      311           
  Lines        7327     7327           
  Branches     1013     1013           
=======================================
  Hits         7051     7051           
  Misses        222      222           
  Partials       54       54           

Flag	Coverage Δ
linux	96.23% <ø> (ø)
macos	96.23% <ø> (ø)
windows	96.22% <ø> (ø)

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