ci: enable test suite for pull requests too (#61)

# Checklist

* [x] I have read the contribution guidelines
* [x] Iff you changed code related to services, or inter-service
communication, make sure you update the diagrams in `ARCHITECTURE.md`.
* [x] Reference issue for this pull request:

# Description

Attempting to enable test suite for every pull request against main.

.github/workflows/build.yml

@@ -1,6 +1,12 @@
 name: build
 
-on: push
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
 
 jobs:
   short-tests: