Initialize Artifact.Workspace to "." by default in plugin case too

[chanseokoh]

Fixes #1803.

[codecov-io]

Codecov Report

Merging #1804 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1804      +/-   ##
==========================================
+ Coverage   45.53%   45.54%   +<.01%     
==========================================
  Files         142      142              
  Lines        6626     6627       +1     
==========================================
+ Hits         3017     3018       +1     
  Misses       3307     3307              
  Partials      302      302

Impacted Files	Coverage Δ
pkg/skaffold/schema/defaults/defaults.go	38.06% <100%> (+0.4%)	⬆️

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