switch to go mod based goveralls in CI

Author: johnbartholomew

Diff for: .github/workflows/ci.yml

@@ -81,8 +81,7 @@ jobs:
         env:
           GO111MODULE: off
         run: |
-          export GOPATH=$GITHUB_WORKSPACE
-          go get github.com/mattn/goveralls
+          go install github.com/mattn/[email protected]
       - name: Send coverage
         env:
           COVERALLS_TOKEN: ${{ secrets.GITHUB_TOKEN }}