When debugging unit tests, use the envFile in settings

[DonJayamanne]

Fixes #1759

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5fa57f8). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1825   +/-   ##
=========================================
  Coverage          ?   74.06%           
=========================================
  Files             ?      282           
  Lines             ?    13142           
  Branches          ?     2357           
=========================================
  Hits              ?     9734           
  Misses            ?     3271           
  Partials          ?      137

Impacted Files	Coverage Δ
src/client/unittests/common/debugLauncher.ts	93.18% <100%> (ø)

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