Use Codecov, coveralls doesn't work at all

The docs of the Github action aren't working at all and are not deemed
to work. The "parallels" feature is completely broken and others have
solved it by switching to Codecov.

See coverallsapp/github-action#13

.github/workflows/main.yml

@@ -63,30 +63,19 @@ jobs:
         make clean
         make -j test-coverage
 
-    - name: Coveralls assemble with lcov
+    - name: Generate coverage report
       run: lcov -c -d . -o lcov.info
       if: "matrix.CC == 'gcc'"
 
-    - name: Coveralls upload
-      uses: coverallsapp/github-action@master
-      if: "matrix.CC == 'gcc'"
+    - name: Upload coverage to Codecov
+      uses: codecov/codecov-action@v1
       with:
-        parallel: true
-        path-to-lcov: "./lcov.info"
-        github-token: ${{ secrets.GITHUB_TOKEN }}
+        token: ${{ secrets.CODECOV_TOKEN }}
+        flags: unittests
+        name: ${{ matrix.distro }}-${{ matrix.CC }}
+        fail_ci_if_error: true
+      if: "matrix.CC == 'gcc'"
 
     runs-on: ubuntu-latest
     container:
       image: dunst/ci:${{ matrix.distro }}
-
-# Just send the signal to coveralls to finish
-# the build and finalise coverage stats
-  finish-coverage:
-    needs: build
-    runs-on: ubuntu-latest
-    steps:
-    - name: Coveralls finish
-      uses: coverallsapp/github-action@master
-      with:
-        parallel-finish: true
-        github-token: ${{ secrets.GITHUB_TOKEN }}