Send parallel builds coverage to coveralls

[DifferentialOrange]

Sometimes different jobs cover different part of code (for example, in #244). To get real coverage results, we need to merge results of different pipelines. coveralls.io documentation says it is possible to merge jobs result, but simply adding a flag doesn't work. (There are many GitHub tickets in other repos saying it doesn't work as expected, like this.)

[DifferentialOrange]

When I've tried to pass the flag, two jobs that reporting to coveralls send their results, but a link to one of the results always responds with 404.

[ligurio]

I suspect that luacov-coveralls should support parallel builds, however I haven't found any mentions of COVERALLS_PARALLEL in its source code.

[ligurio]

We can use luacov reporter to lcov and Github Action for Coveralls that supports parallel builds.