Run coveralls with parallel pipelines

Since introducing CI matrix for `metrics` install, there are more than
one pipeline that should be taken into consideration while computing
coverage. See more in documentation [1].

1. https://docs.coveralls.io/parallel-build-webhook

Follows up #224

.github/workflows/test_on_push.yaml

@@ -68,6 +68,8 @@ jobs:
         run: make -C build coverage
 
       - name: Send code coverage to coveralls.io
+        env:
+          COVERALLS_PARALLEL=true
         run: make -C build coveralls
         if: ${{ matrix.coveralls }}