Move check for plus usage endpoint to test-with-plus target (#2902) (#2904)

[kate-osborn]

Cherry-pick #2902 and #2904 into release-1.5. I'm combining the commits because the first one was an incomplete fix.

Problem: Functional tests for OSS on forks are failing because the PLUS_USAGE_ENDPOINT isn't set.

Solution: Move check for PLUS_USAGE_ENDPOINT to the test-with-plus target.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.71%. Comparing base (9c2deea) to head (160d35b).
Report is 1 commits behind head on release-1.5.

Additional details and impacted files

@@             Coverage Diff              @@
##           release-1.5    #2908   +/-   ##
============================================
  Coverage        89.71%   89.71%           
============================================
  Files              109      109           
  Lines            11091    11091           
  Branches            50       50           
============================================
  Hits              9950     9950           
  Misses            1082     1082           
  Partials            59       59           

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