Debug should report CNB_APP_DIR as working directory for buildpacks images

[briandealwis]

Description

The Cloud Native Buildpacks launcher does a chdir to the directory in the CNB_APP_DIR environment variable (defaulting to /workspace) as the working directory.

This change causes the debug transforms to use and report CNB_APP_DIR as the image working directory in the Container Debug Configuration value that is serialized out in the pod annotation, and which is used to report on debug container events.

(It's surprisingly difficult to set the CNB_APP_DIR — you can't just set it by defining CNB_APP_DIR=xxx during the build!)

[codecov]

Codecov Report

Merging #4326 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4326      +/-   ##
==========================================
+ Coverage   71.76%   71.77%   +0.01%     
==========================================
  Files         324      324              
  Lines       12477    12482       +5     
==========================================
+ Hits         8954     8959       +5     
  Misses       2954     2954              
  Partials      569      569              

Impacted Files	Coverage Δ
pkg/skaffold/debug/cnb.go	92.30% <100.00%> (+0.64%)	⬆️
pkg/skaffold/debug/transform.go	88.77% <100.00%> (+0.06%)	⬆️

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