Optimize k8s-skaffold/skaffold image

[briandealwis]

Rework the gcr.io/k8s-skaffold/skaffold image building process to avoid leaving build artifacts in the image.

[codecov]

Codecov Report

Merging #6106 (f379e6d) into master (044c65f) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6106      +/-   ##
==========================================
- Coverage   70.19%   70.18%   -0.02%     
==========================================
  Files         475      475              
  Lines       18156    18187      +31     
==========================================
+ Hits        12745    12764      +19     
- Misses       4475     4482       +7     
- Partials      936      941       +5     

Impacted Files	Coverage Δ
pkg/skaffold/schema/validation/validation.go	84.44% <0.00%> (-2.62%)	⬇️
pkg/skaffold/server/server.go	41.96% <0.00%> (ø)
pkg/skaffold/schema/latest/v1/config.go	58.82% <0.00%> (ø)
pkg/skaffold/build/cluster/pod.go	86.95% <0.00%> (+0.19%)	⬆️

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