[ci] Pin coveralls runner to 0.6.9

This commit tries to fix the problems we have
with coveralls. It fails and the tests don't
pass on GitHub Actions.

Signed-off-by: Santiago Dueñas <sduenas@bitergia.com>

.github/workflows/tests.yml

@@ -41,4 +41,4 @@ jobs:
           cd tests
           poetry run coverage run --source=perceval run_tests.py
       - name: Coveralls
-        uses: coverallsapp/github-action@f350da2c033043742f89e8c0b7b5145a1616da6d # v2.1.2
+        uses: coverallsapp/github-action@643bc377ffa44ace6394b2b5d0d3950076de9f63 # v2.3.0