Disable CodeCov on commit push (#4572)

As commit push holds write access of GITHUB_TOKEN, we may
want to limit the scope of third-party test run.

This PR disable CodeCov on commit push. Pull request is still
ok.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

.github/workflows/go.coverage.yml

 name: Go Coverage
-on: [push, pull_request]
+on: [pull_request]
 jobs:
   test:
     name: Coverage