Add variable on global HH env identifying solidity-coverage task

[cgewecke]

#678

[codecov-commenter]

Codecov Report

Merging #682 (041199b) into master (5b7eb34) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #682   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          22       22           
  Lines         996      997    +1     
=======================================
+ Hits          983      984    +1     
  Misses         13       13           

Impacted Files	Coverage Δ
plugins/hardhat.plugin.js	93.75% <100.00%> (+0.06%)	⬆️

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