Codecov badge should reflect test coverage of currently used git branch [FR]

[ilyaZar]

Is your feature request related to a problem?

The coverage status badge always reports code coverage for the master branch (as the links are hard-coded) even when switching to a different branch such as dev.

This may be misleading and makes it difficult to (quickly) see the testing coverage when implementing new features to e.g. dev-branch.

Describe the solution you'd like Change the badge links (webpage-link + svg-image-link) inside README.Rmd to the currently used branch (for all branches of {golem}).

Describe alternatives you've considered

• one could scroll down to the "About" section on the README page and see the output of covr::package_coverage() but it takes more time; a badge at the top would make this quicker

Additional context Can be fixed via #1087