Use Coveralls Github Action to send data to coveralls again

This was lost when Travis stopped working, so our coveralls data
is frozen in 2021. This should get it working again using
https://github.com/marketplace/actions/coveralls-github-action if
I got it right.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

.github/workflows/tox.yml

@@ -37,3 +37,7 @@ jobs:
           pip install tox tox-gh-actions
       - name: Test with tox
         run: tox
+      - name: Produce coverage.lcov for coveralls
+        run: coverage lcov
+      - name: Coveralls
+        uses: coverallsapp/github-action@v2