Don't run reviewdog on pushes

As configured, it's meant to annotate PRs with linter errors (so you
don't have to skim through logs). So it does not make sense for pushes.

In fact, on pushes rubocop action is doing nothing, and prettier action
is failing with:

> reviewdog: this is not PullRequest build.
> sed: couldn't write 80 items to stdout: Broken pipe

.github/workflows/linters.yml

@@ -1,5 +1,5 @@
 name: Linters
-on: [push, pull_request]
+on: [pull_request]
 permissions:
   contents: read # to fetch code (actions/checkout)
 jobs: