Refresh `use_github_actions_badge()`

[jennybc]

This commit (hadley/r-pkgs@de6a391) shows the difference between the badge created by use_github_actions_badge() versus what "Create status badge" does via ... in GitHub Actions.

[jennybc]

Seems pretty authoritative re: the general form of the badge links:

https://docs.github.com/en/actions/monitoring-and-troubleshooting-workflows/adding-a-workflow-status-badge

[jennybc]

GitHub will create the markup for a GHA badge like so: visit repo > Actions > specific workflow > ... > Create status badge > Copy status badge Markdown.

For r-lib/usethis and the R-CMD-check workflow, that gives:

[![R-CMD-check](https://github.com/r-lib/usethis/actions/workflows/R-CMD-check.yaml/badge.svg)](https://github.com/r-lib/usethis/actions/workflows/R-CMD-check.yaml)

Let's compare that to what use_github_actions_badge() currently does.

In terms of the URL for the .svg:

we currently do: https://github.com/r-lib/usethis/workflows/R-CMD-check/badge.svg
GitHub suggests: https://github.com/r-lib/usethis/actions/workflows/R-CMD-check.yaml/badge.svg

To be clear, our .svg URLs do work, but it's probably best to fall in line with what seems to be more official / current.

In terms of what the badge links to:

we currently do: https://github.com/r-lib/usethis/actions
GitHub suggests: https://github.com/r-lib/usethis/actions/workflows/R-CMD-check.yaml

Both links take you to a repo's GitHub Actions page, but GitHub's more specific link pre-filters the runs that you see in the main right-hand panel. It's still very easy to click around to other workflows, e.g. pkgdown or test-coverage, in the left-hand panel.

I think use_github_actions_badge() should sync up with what the "GHA badge helper" suggests.