Extract push/no-push logic into builder

[dgageot]

Minor refactoring on Jib builders to make their code look more like the other's.
This is to prepare work on how builders tag images.

Signed-off-by: David Gageot david@gageot.net

[codecov-io]

Codecov Report

Merging #1366 into master will decrease coverage by 0.03%.
The diff coverage is 38.23%.

@@            Coverage Diff             @@
##           master    #1366      +/-   ##
==========================================
- Coverage   44.95%   44.91%   -0.04%     
==========================================
  Files         114      114              
  Lines        4921     4925       +4     
==========================================
  Hits         2212     2212              
- Misses       2482     2486       +4     
  Partials      227      227

Impacted Files	Coverage Δ
pkg/skaffold/build/local/local.go	51.78% <0%> (+3.45%)	⬆️
pkg/skaffold/build/local/jib_gradle.go	21.87% <36.36%> (-3.13%)	⬇️
pkg/skaffold/build/local/jib_maven.go	38.59% <42.85%> (-2.92%)	⬇️

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