Coveralls reports "new coverage" for a matrix build in Github Actions for a PR for master branch

[tomato42]

I've configured a Github Actions matrix build, and while coveralls recognises the build as a parallel and a PR, it doesn't recognise it as a PR for a master branch, so it reports it as "new coverage" not a coverage change with existing builds.

See the PR here:
tlsfuzzer/tlslite-ng#449
coveralls report for it here:
https://coveralls.io/builds/36466430
and a coveralls report for a previous master branch build here:
https://coveralls.io/builds/35554556
and an old travis PR run for comparison:
https://coveralls.io/builds/35832561