Github actions fail on scheduled workflows

When you try to run mix coveralls.github in workflows that are scheduled using cron schedule (https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule) you get a following error:

** (BadMapError) expected a map, got: nil
    (elixir 1.13.3) lib/map.ex:481: Map.get(nil, "login", nil)
Warning:     (excoveralls 0.14.4) lib/excoveralls/github.ex:93: ExCoveralls.Github.git_info/0
    (excoveralls 0.14.4) lib/excoveralls/github.ex:25: ExCoveralls.Github.generate_json/2
Warning:     (excoveralls 0.14.4) lib/excoveralls/github.ex:8: ExCoveralls.Github.execute/2
    (mix 1.13.3) lib/mix/tasks/test.ex:538: Mix.Tasks.Test.do_run/3
Warning:     (mix 1.13.3) lib/mix/task.ex:397: anonymous fn/3 in Mix.Task.run_task/3
    (mix 1.13.3) lib/mix/task.ex:451: Mix.Task.run_alias/5
Warning:     (mix 1.13.3) lib/mix/task.ex:455: Mix.Task.run_alias/5
    (excoveralls 0.14.4) lib/mix/tasks.ex:54: Mix.Tasks.Coveralls.do_run/2
Warning:     (mix 1.13.3) lib/mix/task.ex:397: anonymous fn/3 in Mix.Task.run_task/3
    (mix 1.13.3) lib/mix/cli.ex:84: Mix.CLI.run_task/2
Warning:     (elixir 1.13.3) lib/code.ex:1183: Code.require_file/2

The reason for this is we are trying to read GITHUB_EVENT_PATH file which contains event payload, but the payload doesn't contain anything much in workflows scheduled like this. The file contains basically something like this only:

{
  "schedule": "*/10 * * * *"
}

To fix this we should fallback to other ways of getting information for functions:

get_pr_id
get_committer_name
get_sha

parroty / excoveralls

Github actions fail on scheduled workflows #280