chore(backport release-1.2): fix: fix faulty logic for handling possible double-encoding of github app private key

[akuitybot]

Automated backport to release-1.2, triggered by a label in #3363.

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[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 52.50%. Comparing base (cf51d11) to head (c8e764a).
Report is 1 commits behind head on release-1.2.

Files with missing lines	Patch %	Lines
internal/credentials/kubernetes/github/app.go	68.75%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-1.2    #3379      +/-   ##
===============================================
+ Coverage        52.48%   52.50%   +0.01%     
===============================================
  Files              291      291              
  Lines            26613    26625      +12     
===============================================
+ Hits             13968    13979      +11     
  Misses           11882    11882              
- Partials           763      764       +1     

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