Skip plus tests on a fork

[sjberman]

Problem: The NGINX Plus JWT is only available on the upstream repo, so automated tests for Plus can't be run on forks.

Solution: Skip these test runs on a fork.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.86%. Comparing base (581fd74) to head (0e682ff).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2784   +/-   ##
=======================================
  Coverage   89.86%   89.86%           
=======================================
  Files         107      107           
  Lines       10997    10997           
  Branches       50       50           
=======================================
  Hits         9883     9883           
  Misses       1054     1054           
  Partials       60       60           

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