ci: split debug_coverage.yml into two workflows

This change splits the debug_coverage.yml workflow into two independent
workflows: debug.yml and coverage.yml.

The first one will be run on push to a branch and creating/updating
external pool requests. In other words, it will be run always.

The second one will be run on push to a branch only if the branch has
the `full-ci` suffix (some-branch-full-ci) or the `full-ci` label is set
on a pool request, internal or external - it doesn't matter.

Why do we need it?

1. The coverage testing is heavy testing due to enabling long tests
   and takes at least 2 times more time than debug build testing. So
   testing that is done at the first stage (lint, release, debug) will
   pass faster.
2. The coverage report as a pool request comment from coveralls.io can
   be obtained only if a pool request event happened. It is a limitation
   of the `coverallsapp/github-action` action [1]. So there is no sense
   to run the coverage workflow on push events. But the coverage report
   on push still can be seen on coveralls.io if the branch has the
   `full-ci` suffix.
3. The logic of the debug_coverage.yml workflow is tricky enough and no
   one understands how it works. So we would like to have an absolutely
   transparent and simple workflow.

[1] https://github.com/coverallsapp/github-action/tree/8cbef1dea373ebce56de0a14c68d6267baa10b44#coveralls-github-action

Closes tarantool/tarantool-qa#147

NO_DOC=ci
NO_CHANGELOG=ci