Default to empty secret path for Kaniko to use Workload Identity credentials

[chtcvl]

Fixes: #3468

Description
If GOOGLE_APPLICATION_CREDENTIALS is set, Kaniko doesn't opt for using Workload Identity. Current setup always forces non-empty GOOGLE_APPLICATION_CREDENTIALS value. Proposed change make empty value default for pullSecretPath, unless MountPath is specified in the config. This in turn prevents GOOGLE_APPLICATION_CREDENTIALS from being set.

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[chtcvl]

@googlebot I signed it!

[google-cla]

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[chtcvl]

@googlebot I signed it!

[tejal29]

Please rebase!

[codecov]

Codecov Report

Merging #5730 (cbcc43c) into master (ce34201) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5730   +/-   ##
=======================================
  Coverage   70.95%   70.95%           
=======================================
  Files         439      439           
  Lines       16408    16410    +2     
=======================================
+ Hits        11642    11644    +2     
  Misses       3913     3913           
  Partials      853      853           

Impacted Files	Coverage Δ
pkg/skaffold/build/cluster/pod.go	86.76% <100.00%> (+0.19%)	⬆️

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