Support go Templates in Custom Builder commands

[dgageot]

Fixes #3753

[codecov]

Codecov Report

Merging #3754 into master will increase coverage by 0.01%.
The diff coverage is 84.21%.

Impacted Files	Coverage Δ
pkg/skaffold/docker/image.go	73.89% <100%> (+0.53%)	⬆️
pkg/skaffold/util/env_template.go	100% <100%> (ø)	⬆️
pkg/skaffold/build/misc/env.go	100% <100%> (+12.5%)	⬆️
pkg/skaffold/deploy/helm.go	77.47% <100%> (+0.29%)	⬆️
pkg/skaffold/build/custom/script.go	52.38% <40%> (-1.47%)	⬇️
...affold/kubernetes/portforward/kubectl_forwarder.go	65.85% <0%> (-2.44%)	⬇️
pkg/skaffold/sync/sync.go	72.57% <0%> (+0.31%)	⬆️

[nkubala]

nice!