better github action support?

[kyungminlee]

add_ci_to_kwargs from codecovio.jl can be updated to provide better support for GitHub Action. Current version ignores branch when submitting to CodeCov.

Coverage.jl/src/codecovio.jl

Lines 142 to 147 in 9ddfeaf

	elseif haskey(ENV, "GITHUB_ACTION") # GitHub Actions
	kwargs = set_defaults(kwargs,
	service = "custom",
	commit = ENV["GITHUB_SHA"],
	slug = ENV["GITHUB_REPOSITORY"],
	)

[fingolfin]

Unfortunately nobody is really working on Coverage.jl right now. If you have an inkling of what needs to be done here (e.g. how to figure out the correct branch and how to submit it), perhaps consider submitting a PR to Coverage.jl to get it done?