chore: optimize ci (#481)

* chore: prevent ci from running twice on PRs

Now CI will only run on push to `master` and PRs to `master`. Previously, it was
running twice on each push to a PR branch, once for the PR to `master` and once 
for pushing to the PR branch.

.github/workflows/ci.yml

 name: ci
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
   cancel-in-progress: true