ci: run tests on push to staging branch

Bors will push merge commits to a branch called 'staging'. This push
should trigger the CI to run. Bors will then await the completion of
that run.

.github/workflows/build-test.yml

@@ -1,6 +1,9 @@
 name: Build and test
 on:
   pull_request: { }
+  push:
+    branches:
+      - 'staging'
   workflow_call: { }
 jobs:
   code-formatting: