Trigger Github Actions only once per commit

Previously, CI via Github Actions would be triggered multiple times per
push if there is an open PR for the corresponding branch. When a
developer pushes a new commit, the first run would be triggered by the
`push` event, and the second run triggered by the `pull_request` event
(because pushing new commits _alters_ the pull request).

.github/workflows/ci.yml

@@ -1,8 +1,6 @@
 name: build
 on:
   push:
-  pull_request:
-  workflow_dispatch:
 jobs:
   build:
     runs-on: ubuntu-latest