Limit push builds to the main branch (#1730)

GitHub Actions has been running the lint twice in Pull Requests: once
following the `on push` trigger, once following `on pull_request`. This
duplication is currently avoided when running tests by an `if` rule,
which skips if the event isn't a push or the Pull Request doesn't come
from a fork.

This commit removes the `if` rule, and limits the `push` builds to the
`main` branch. That will ensure that merged Pull Requests will trigger a
build, and that any Pull Request activity will also trigger a build.
Being a global rule for the whole build, `standard` will not run twice
anymore.

.github/workflows/build.yml

@@ -1,12 +1,12 @@
 name: Build
 on:
-  - push
-  - pull_request
+  pull_request:
+  push:
+    branches: [ main ]
 
 jobs:
   build:
     name: Ruby ${{ matrix.ruby }} / Rails ${{ matrix.rails }}
-    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
     strategy:
       fail-fast: false
       matrix: