Run Continuous integration workflow on PRs

Make the workflow get triggered either by pushes to the default branch, by
pull requests targeting the default branch, or by manually triggering it from
Github's UI.

.github/workflows/workflow.yaml

@@ -1,5 +1,12 @@
 name: Continuous integration
-on: push
+
+on:
+  push:
+    branches: master
+  pull_requests:
+    branches: master
+  workflow_dispatch: # allows manual triggering
+
 jobs:
   test-nixpkgs:
     name: Build & Test - Nixpkgs