Drone coverage fix (#2592)

* Build as nobody

This showld fix coverage files writing. At least some coverage files
(*.gcda, *.gcno) are written by rspamd running as nobody.

* Comment luacov-coveralls

* Run rspamd-test as nobody too

* Run coveralls from build root (/rspamd/build)

It need to find all `.gcda` files.

* Hack for coveralls was moved to Docker image

* Add test debug

* More debug

* Workaround to writing *.gcda from different users

*.gcda files written in parallel by root and by nobody

* Remove debug

* Use current build number instead of previous

* Remove unused variable

* Add more parameters

- If CI detected we don't need to check if service_job_id and other
fields in json already filled - this info anyway comes from environment

- coveralls.io API is not documented. Code from:
https://github.com/lemurheavy/coveralls-ruby/
and
https://github.com/coveralls-clients/coveralls-python/
was used as an example.

* In case of error there is no url in json respnse

Print message field anyway - it can contain useful error message.

* Add debug

* Simplify

* Fix: for pull request CI_BUILD_EVENT set to 'pull_request'

* Remove debug

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