Add support for CODECOV_TOKEN

[HDash]

Add support for the CODECOV_TOKEN GitHub secret. This input helps prevent coverage report upload failures, which have been quite frequent recently.

Coverage report upload failure error:

Run covr::codecov()
  covr::codecov()
  shell: Rscript {0}
  env:
    RGL_USE_NULL: TRUE
    R_REMOTES_NO_ERRORS_FROM_WARNINGS: true
    RSPM: https://packagemanager.rstudio.com/cran/__linux__/focal/release
    TZ: UTC
    NOT_CRAN: false
Request failed [429]. Retrying in 1 seconds...
Request failed [429]. Retrying in 2.5 seconds...
$detail
[1] "Rate limit reached. Please upload with the Codecov repository upload token to resolve issue. Expected time to availability: 1915s."

More information: https://docs.codecov.com/docs/adding-the-codecov-token

This implementation has been successfully tested on MotifPeeker with my fork.

[bschilder]

Thanks so much for this @HDash !