Fix unnecessary warning in caching

[priyawadhwa]

If an image is built remotely, we don't want to try to retag/push it locally, otherwise we get an unnecessary warning that caching may not work.

(I know we haven't finished deciding what to do with caching for spring cleanup yet, but we need this fix for the demo me and @dgageot are working on!) :)

[priyawadhwa]

thanks @balopat !

[codecov-io]

Codecov Report

Merging #1873 into master will increase coverage by 0.08%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1873      +/-   ##
==========================================
+ Coverage   50.16%   50.24%   +0.08%     
==========================================
  Files         168      168              
  Lines        7356     7354       -2     
==========================================
+ Hits         3690     3695       +5     
+ Misses       3310     3300      -10     
- Partials      356      359       +3

Impacted Files	Coverage Δ
pkg/skaffold/build/cache/cache.go	67.34% <100%> (+1.38%)	⬆️
pkg/skaffold/build/cache/retrieve.go	86.82% <100%> (ø)	⬆️
pkg/skaffold/build/cache/save.go	20.68% <33.33%> (+9.97%)	⬆️
pkg/skaffold/runner/runner.go	61.99% <80%> (+0.31%)	⬆️

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[balopat]

I understand the urgency around the demo - but I think we shouldn't merge this without putting a test around the failing use case. I might be able to help with that too, if you're okay with me meddling in your PR.

[priyawadhwa]

@balopat don't mind at all! I wasn't exactly sure what kind of test to write, since the retagging wasn't failing, it was just unnecessary.

I changed the name of the function to RetagLocalImages to make it clear it should be skipped if images aren't built locally, and put in a unit test for the retagging.