actions: prevent workflows triggering twice

the workflows running unconditionally for each push and each PR are run
twice when pushing to a branch that is to be merged by a PR and if that
branch is a branch local to the repo (rather than in a fork).

.github/workflows/cpplint.yml

@@ -6,6 +6,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
 
+    # prevent push event from triggering if it's part of a PR
+    if: github.event_name != 'push' || github.event.pull_request == null
+
     steps:
       - uses: actions/checkout@v4
       - name: Set up Python

.github/workflows/yarnlint.yml

@@ -6,6 +6,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
 
+    # prevent push event from triggering if it's part of a PR
+    if: github.event_name != 'push' || github.event.pull_request == null
+
     defaults:
       run:
         working-directory: webapp

.github/workflows/yarnprettier.yml

@@ -6,6 +6,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
 
+    # prevent push event from triggering if it's part of a PR
+    if: github.event_name != 'push' || github.event.pull_request == null
+
     defaults:
       run:
         working-directory: webapp