Run GitHub Actions checks on pull_request rather than push event

This ensures that GitHub Actions checks run for PRs submitted from forks,
which includes all PRs submitted by people outside the Hypothesis
organization.

Note that this change which commit the checks run on. See [1] for
details.

This change also matches how GitHub CI is set up in other projects (eg.
lms, h).

[1] https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request

.github/workflows/continuous-integration.yml

 name: Continuous integration
-on: [push]
+on: pull_request
 jobs:
   ci:
     runs-on: ubuntu-latest