Update schedule to only run GHA on pull requests (the unit tests specifically only run when src/** or tests/** change)

Author: JamieJQuinn

.github/workflows/build_and_test.yml

@@ -1,7 +1,10 @@
 name: Run unit tests
 
 on:
-  push:
+  pull_request:
+    paths:
+      - 'src/**'
+      - 'tests/**'
 
 jobs:
   unit-tests:

.github/workflows/check_policies.yml

@@ -1,5 +1,7 @@
 name: check-policies
-on: [push]
+
+on: [pull_request]
+
 jobs:
   check-commit-messages:
     runs-on: ubuntu-latest
@@ -14,6 +16,7 @@ jobs:
       - run: |
           bash .github/workflows/check_commit_messages.sh \
             ${{ github.event.before }} ${{ github.event.after }}
+
   check-formatting:
     runs-on: ubuntu-latest
     steps: