chore(backport release-1.1): fix: fix faulty logic for handling possible double-encoding of github app private key

[akuitybot]

Automated backport to release-1.1, 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 51.19%. Comparing base (5c875fe) to head (f2e3445).
Report is 1 commits behind head on release-1.1.

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-1.1    #3378      +/-   ##
===============================================
+ Coverage        51.17%   51.19%   +0.01%     
===============================================
  Files              285      285              
  Lines            25627    25639      +12     
===============================================
+ Hits             13115    13126      +11     
  Misses           11814    11814              
- Partials           698      699       +1     

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