Upload sources for any kind of artifact

[dgageot]

This is needed if we want to build Jib or Bazel artifacts on GCB

[codecov-io]

Codecov Report

Merging #1477 into master will increase coverage by 0.38%.
The diff coverage is 57.57%.

@@            Coverage Diff            @@
##           master   #1477      +/-   ##
=========================================
+ Coverage   44.42%   44.8%   +0.38%     
=========================================
  Files         112     112              
  Lines        4642    4638       -4     
=========================================
+ Hits         2062    2078      +16     
+ Misses       2373    2350      -23     
- Partials      207     210       +3

Impacted Files	Coverage Δ
pkg/skaffold/build/deps.go	0% <ø> (ø)
pkg/skaffold/docker/context.go	71.42% <ø> (+40.39%)	⬆️
pkg/skaffold/build/gcb/cloud_build.go	0% <0%> (ø)	⬆️
pkg/skaffold/runner/dev.go	56.52% <0%> (ø)	⬆️
cmd/skaffold/app/cmd/diagnose.go	0% <0%> (ø)	⬆️
pkg/skaffold/util/tar.go	38.46% <62.5%> (+21.48%)	⬆️
pkg/skaffold/docker/parse.go	74.88% <66.66%> (-0.23%)	⬇️

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[priyawadhwa]

LGTM