Add PR workflow action trigger

I modified some settings inside the repository based on
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#setting-the-permissions-of-the-github_token-for-your-repository
and
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#allowing-select-actions-and-reusable-workflows-to-run

for security.

.github/workflows/build-test.yaml

@@ -6,6 +6,7 @@ name: build-test
 on: 
   push:
   workflow_dispatch:
+  pull_request:
 jobs:
   validate:
     name: Format and Lint Check