ci: build pull requests too

The workflow only ran on push, so a pull request opened from a fork
never got a build, and neither did the host-side unit tests that run in
the same job.

.github/workflows/build.yml

@@ -3,6 +3,7 @@ name: Build firmware
 on:
   push:
     branches: ['**']
+  pull_request:
 
 jobs:
   build: