Don't build PRs until we have a working pipeline

Avoid building each commit twice while we're figuring out the CI
pipeline. We should revert this change before merging this PR into the
main branch.

.github/workflows/build.yml

@@ -1,6 +1,6 @@
 name: CI
 
-on: [push, pull_request]
+on: [push] # FIXME: Temporarily avoid building PRs while setting up the CI
 
 jobs:
   test: