gh-978: Escape the pipenv executable for activation.

[ericsnowcurrently]

(for #978)

I missed a corner case in #3957. This fixes it.

[codecov]

Codecov Report

Merging #3975 into master will increase coverage by 1%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #3975   +/-   ##
======================================
+ Coverage      78%     78%   +1%     
======================================
  Files         406     406           
  Lines       18892   18892           
  Branches     3028    3028           
======================================
+ Hits        14642   14644    +2     
+ Misses       4248    4246    -2     
  Partials        2       2

Flag	Coverage Δ
#Linux	68% <0%> (+1%)	⬆️
#Windows	68% <0%> (+1%)	⬆️
#macOS	68% <0%> (ø)	⬇️

Impacted Files	Coverage Δ
...entActivationProviders/pipEnvActivationProvider.ts	79% <100%> (ø)	⬆️
src/client/interpreter/helpers.ts	79% <0%> (-1%)	⬇️
src/client/common/platform/fileSystem.ts	64% <0%> (ø)	⬇️
src/client/providers/jediProxy.ts	77% <0%> (ø)	⬇️
src/client/common/process/pythonProcess.ts	93% <0%> (+2%)	⬆️
...rpreter/locators/services/baseVirtualEnvService.ts	88% <0%> (+3%)	⬆️
...tors/services/workspaceVirtualEnvWatcherService.ts	90% <0%> (+5%)	⬆️

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[ericsnowcurrently]

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