Fix coveralls integration

See TheKevJames/coveralls-python#252: the
coveralls API requires a service name, which used to be 'github', but a
new release of https://pypi.org/p/coveralls switched the default to
'github-actions'.

Nobody understands the exact difference between the two, but I have 19
repositories that now fail coveralls uploads using the 'github-actions'
service name, and I hope that switching to 'github' will fix them.

.github/workflows/build.yml

@@ -65,6 +65,7 @@ jobs:
         if: "matrix.python-version != '2.7' && matrix.python-version != 'pypy2'"
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          COVERALLS_SERVICE_NAME: github
 
   lint:
     name: ${{ matrix.toxenv }}