update ci for modern way of installing goverall

Author: umputun

.github/workflows/ci.yml

@@ -31,7 +31,8 @@ jobs:
           version: latest
 
       - name: install goveralls
-        run: GO111MODULE=off go get -u -v github.com/mattn/goveralls
+        run: |
+          go install github.com/mattn/goveralls@latest
 
       - name: submit coverage
         run: $(go env GOPATH)/bin/goveralls -service="github" -coverprofile=$GITHUB_WORKSPACE/profile.cov