Workflow 'debug_coverage' produces and uploads results to 'coveralls.io'
web site. Message about it can be shown in PR within each run was done.
This patch adds the ability to send message in available PR otherwise it
is skipped. Also added 'coverage.info' file to artifacts list.

Found that used 'coverallsapp/github-action' action checks if the
trigger was 'pull_request' used to run the workflow [1]. And only in
this way it writes results message to PR. Previously 'pull_request'
trigger was blocked to avoid duplication with 'push' trigger. To make
able to run workflow by any single trigger and to send message to PR
if it exits, it was implemented the following logic:

  - run workflows on both triggers 'push' and 'pull_request';
  - for workflow with 'push' trigger check if PR exists then skip all
    jobs otherwise continue running;
  - for workflow with 'pull_request' trigger continue running;

To avoid of issue coverallsapp/github-action#55 [2], sources checkout
should be based on 2nd version and later.

Closes #5644

[1]: https://github.com/coverallsapp/github-action/blob/master/src/run.ts#L38
[2]: https://github.com/coverallsapp/github-action/issues/55#issuecomment-644927165