Use CODECOV_TOKEN to fix code coverage uploads

[jdblischak]

Our code coverage results have not been uploaded for months because we aren't using a Codecov token. We get errors like the following from our most recent run on main:

[1] "Rate limit reached. Please upload with the Codecov repository upload token to resolve issue. Expected time to availability: 2836s."

We were recently provided a single repository upload token for the gsDesign2 repository by the Merck open source team. Once it is added as a repository secret named CODECOV_TOKEN, our workflow will start uploading coverage results again.

xref: r-lib/actions#834

[nanxstats]

@jdblischak Should we just run usethis::use_github_action("test-coverage") to update to the latest test-coverage workflow from r-lib/actions? There seems to be some accumulated changes compared to the old workflow we are using.

[jdblischak]

I confirmed in a manually triggered run of an internal branch that CODECOV_TOKEN has been set as a repository secret and can successfully upload the report

https://github.com/Merck/gsDesign2/actions/runs/12053747467/job/33610137275#step:5:16
https://github.com/Merck/gsDesign2/actions/runs/12053747467/job/33610137275#step:5:1016
https://app.codecov.io/github/Merck/gsDesign2/commit/f8055857d3f5179fbe96fc8c29b04d734006c81b

[jdblischak]

Repository secrets are not exposed to pull requests from forks, so it doesn't work by default in this case. However, when I updated to the latest test-coverage.yaml, it uses a new action that uses the token by default but then attempts tokenless uploads for pull requests. And surprisingly the tokenless upload appears to have worked. The action is official from Codecov, so maybe it allows tokenless uploads for Pull Requests. Not sure.

https://github.com/Merck/gsDesign2/actions/runs/12057253346/job/33621476108#step:6:53
https://app.codecov.io/github/Merck/gsDesign2/commit/2c2f57ab28d548bfb0bfb9885c948b02f85f0085

[jdblischak]

Ready for review!