ci: [GHA] upload coverage report

Upload the coverage report as an artifact.  We still need to work
out what we can actually *do* with it on GitHub which, unlike
GitLab, has no native support for coverage annotations in PRs.
But we have agreed to solve that another day.

.github/workflows/main.yml

@@ -57,7 +57,10 @@ jobs:
           go run github.com/boumenot/gocover-cobertura < coverage.out > coverage.xml
           go tool cover -func ./coverage.out
 
-      # TODO upload coverage report as artifact
+      - name: Upload coverage report
+        uses: "actions/upload-artifact@v3"
+        with:
+          path: coverage.xml
 
       - name: Build executables
         run: make build