Disable CodeCov upload in tests on forks

[charlesbluca]

In general we should be doing this, as this step would always fail if we run the PR testing on forks; this should make it easier to manage a RAPIDS fork of this repo with minimal changes

[codecov-commenter]

Codecov Report

Merging #349 (f0fe136) into main (476c354) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #349   +/-   ##
=======================================
  Coverage   95.68%   95.68%           
=======================================
  Files          66       66           
  Lines        2898     2898           
  Branches      542      542           
=======================================
  Hits         2773     2773           
  Misses         75       75           
  Partials       50       50           

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