Temporarily disable Test coverage step on GitHub Action CI

[yutannihilation]

covr::codecov() keeps failing on GitHub Action CI.

Error in curl::curl_fetch_memory(url, handle = handle) : 
  HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)
Calls: <Anonymous> ... request_fetch -> request_fetch.write_memory -> <Anonymous>
Execution halted
##[error]Process completed with exit code 1.

https://github.com/tidyverse/ggplot2/runs/509138337?check_suite_focus=true#step:13:14

I tried some tweaks on my forked repo (yutannihilation#4), but none of these worked. As the other repos (e.g. roxygen2) works without error, this seems specific to ggplot2. So, I think it takes some time to figure out what's happening here, so this should be disabled for now.

Fortunately(?), I forgot to remove the test coverage step on Travis CI in #3862, so we can rely on it for a while.

[yutannihilation]

Thanks for approving.