Fix PickFile and PickDirectory

[manandre]

Fixes #1003

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 8.14%. Comparing base (5c733cb) to head (80abfa3).

Files	Patch %	Lines
src/Nuke/FilePathExtensions.cs	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1232      +/-   ##
=========================================
- Coverage    8.19%   8.14%   -0.05%     
=========================================
  Files         146     146              
  Lines        3638    3632       -6     
  Branches      287     285       -2     
=========================================
- Hits          298     296       -2     
+ Misses       3340    3336       -4     

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[david-driscoll]

Thanks for the find @manandre !