Fix GithubIntegration that uses expiring jwt

[EnricoMi]

Pull request #1986 substantially rewrote GithubIntegration, which lost using a fresh jwt token when calling get_access_token. This reverts that change.

Fixes #2431.

This does not fix expiring JWT tokens when iterating over pages returned by get_installations(), or calling get_repos() on returned Installations. This will require a bigger rework.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (300c501) 98.77% compared to head (d96557b) 98.77%.

❗ Current head d96557b differs from pull request most recent head 45e026e. Consider uploading reports for the commit 45e026e to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2460   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files         117      117           
  Lines       11674    11674           
=======================================
  Hits        11531    11531           
  Misses        143      143           

Impacted Files	Coverage Δ
github/GithubIntegration.py	96.61% <ø> (ø)

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[JLLeitschuh]

LGTM!

[EnricoMi]

@s-t-e-v-e-n-k can you please approve?