Multiple small improvements to unit tests

[dgageot]

• Simplify the testutil.SetEnv() helper
• Use testutil.Override where possible
• Use util.OSEnviron() everywhere

[codecov-io]

Codecov Report

Merging #2189 into master will not change coverage.
The diff coverage is 25%.

@@           Coverage Diff           @@
##           master    #2189   +/-   ##
=======================================
  Coverage   57.91%   57.91%           
=======================================
  Files         187      187           
  Lines        7900     7900           
=======================================
  Hits         4575     4575           
  Misses       2912     2912           
  Partials      413      413

Impacted Files	Coverage Δ
pkg/skaffold/util/env_template.go	76.47% <ø> (ø)	⬆️
pkg/skaffold/build/local/docker.go	53.19% <0%> (ø)	⬆️
pkg/skaffold/build/local/jib_maven.go	26.66% <0%> (ø)	⬆️
pkg/skaffold/build/local/jib_gradle.go	0% <0%> (ø)	⬆️
pkg/skaffold/build/custom/custom.go	66.66% <100%> (ø)	⬆️

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