Propagate kaniko environment to GCB

[briandealwis]

Fixes: #6174

[codecov]

Codecov Report

Merging #6181 (4342a84) into master (37cdc30) will increase coverage by 0.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #6181      +/-   ##
==========================================
+ Coverage   71.06%   71.08%   +0.01%     
==========================================
  Files         482      482              
  Lines       21517    21526       +9     
==========================================
+ Hits        15291    15301      +10     
  Misses       5247     5247              
+ Partials      979      978       -1     

Impacted Files	Coverage Δ
pkg/skaffold/build/gcb/kaniko.go	76.00% <77.77%> (+1.00%)	⬆️
pkg/skaffold/docker/image.go	81.30% <0.00%> (+1.21%)	⬆️

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