Fix duplicated owner when calling GitHub APIs

[ArthurSens]

While working on enabling issuegenerator for the collector-contrib repo(open-telemetry/opentelemetry-collector-contrib#38270), I realized we were duplicating the github owner when calling GitHub APIs.

After rereading the docs, I realized that the environment variable GITHUB_REPOSITORY already holds the owner information instead of just the repository information. This PR adjusts the usage of those environment variables, making sure we don't duplicate the owner when calling GitHub APIs.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 52.26%. Comparing base (b62c325) to head (3f3f15d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
issuegenerator/main.go	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #720      +/-   ##
==========================================
- Coverage   52.32%   52.26%   -0.06%     
==========================================
  Files          59       59              
  Lines        3524     3528       +4     
==========================================
  Hits         1844     1844              
- Misses       1501     1505       +4     
  Partials      179      179              

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