Do not upload coverage results to Codecov when run in a fork

[jdblischak]

When I sync my fork (eg today in jdblischak@98f1234), the test-coverage workflow fails because I don't have CODECOV_TOKEN set.

==> Running command '/home/runner/work/_actions/codecov/codecov-action/v4/dist/codecov create-commit'
/home/runner/work/_actions/codecov/codecov-action/v4/dist/codecov create-commit --git-service github -Z
info - 2025-01-27 19:45:39,664 -- ci service found: github-actions
warning - 2025-01-27 19:45:[39](https://github.com/jdblischak/gsDesign2/actions/runs/12996942372/job/36246800050#step:6:40),694 -- Branch `main` is protected but no token was provided
warning - 2025-01-27 19:45:39,694 -- For information on Codecov upload tokens, see https://docs.codecov.com/docs/codecov-tokens
info - 2025-01-27 19:45:[40](https://github.com/jdblischak/gsDesign2/actions/runs/12996942372/job/36246800050#step:6:41),169 -- Process Commit creating complete
error - 2025-01-27 19:45:40,169 -- Commit creating failed: {"message":"Token required because branch is protected"}

We only need coverage uploads to Codecov to run on the main repository, so this PR updates the workflow to skip the upload step when running on a fork. However, when running in a PR submitted from a fork, it still performs token-less uploads to Codecov.

xref: #489, Merck/simtrial#307, keaven/gsDesign#179

[jdblischak]

The token-less upload from this PR succeeded:

https://github.com/Merck/gsDesign2/actions/runs/12997143920/job/36247473608?pr=496#step:6:53
https://app.codecov.io/github/Merck/gsDesign2/commit/3c60081c96585f21a8f262d59132c7ca28205a5f

[LittleBeannie]

Thanks, @jdblischak!

[jdblischak]

I confirmed it worked.

On the merge commit in this repo, ff9a1d5, the coverage was successfully uploaded to Codecov:

https://github.com/Merck/gsDesign2/actions/runs/13016835298/job/36308040811#step:6:52
https://app.codecov.io/github/Merck/gsDesign2/commit/ff9a1d5e6963c7009ce56ee8e2ec94594c97faa4

And when I synced my fork, jdblischak@ff9a1d5, the upload step was skipped:

https://github.com/jdblischak/gsDesign2/actions/runs/13017000281/job/36308570223