Gatling launcher script logic update fix for windows

[schneidh]

Fix for the gatling launcher script update logic for windows. It currently does not update the launcher script correctly for the set CLASSPATH= case. (It looks like the unix logic was copied and not updated)

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Each PR must conform to Developer's Guide.

Quick checklist:

• Description of PR explains the context of change
• Unit tests cover the change, no broken tests
• No static analysis warnings (Codacy etc.)
• Documentation update ('available in the unstable snapshot' warning if necessary)
• Changes file inside site/dat/docs/changes directory, one-line note of change inside

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.86%. Comparing base (09be9b6) to head (391c4ee).
Report is 2 commits behind head on master.

Files	Patch %	Lines
bzt/modules/gatling.py	90.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1848      +/-   ##
==========================================
+ Coverage   89.80%   89.86%   +0.07%     
==========================================
  Files          71       71              
  Lines       19538    19544       +6     
==========================================
+ Hits        17544    17562      +18     
+ Misses       1994     1982      -12     

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