CNB command-line should only be rewritten if changed

[briandealwis]

Fixes: #4166

Description

In skaffold debug we synthesize an imageConfiguration object for an image containing the environment variables, image labels, command-line, and working directory, as derived from the container image's configuration and the corresponding Kubernetes manifest's container settings.
For buildpacks images, we swizzle the image configuration by resolving the corresponding process definition and replacing the command-line with the process definition, perform the normal debug transforms, and then unswizzle as required for the CNB launching mechanism.

But some of our debug transforms don't actually change the command-line, and so our unswizzling performs a rewrite on a seemingly empty command-line, resulting in a broken command-line.

This change only unswizzles (or rewrites) the container's command-line if it was set. If unset, then we'll use the command-line in the container image's configuration.

[codecov]

Codecov Report

Merging #4176 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
pkg/skaffold/debug/cnb.go	91.66% <100.00%> (ø)
pkg/skaffold/event/event.go	92.53% <0.00%> (-1.17%)	⬇️
pkg/skaffold/deploy/status_check.go	51.87% <0.00%> (+0.40%)	⬆️