Fix 'envFile' launch.json option on Windows

[gregg-miskelly]

The 'workspaceFolder' subsitution logic was wrong for 'envFile', which broke the feature on Windows. This fixes it.

This fixes #2512

[codecov]

Codecov Report

Merging #2560 into master will increase coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2560      +/-   ##
==========================================
+ Coverage   65.04%   65.09%   +0.04%     
==========================================
  Files          97       97              
  Lines        4226     4226              
  Branches      605      605              
==========================================
+ Hits         2749     2751       +2     
+ Misses       1304     1303       -1     
+ Partials      173      172       -1

Flag	Coverage Δ
#integration	52.92% <0%> (+0.04%)	⬆️
#unit	84.76% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/configurationProvider.ts	21.42% <0%> (ø)	⬆️
src/omnisharp/delayTracker.ts	73.68% <0%> (+5.26%)	⬆️

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