Runs lint on all PRs to main (including external contributors) (#1502)

* Update lint.yml

* Build and e2e on PRs

.github/workflows/build.yml

@@ -1,5 +1,12 @@
 name: Build
-on: [push]
+on:
+  push:
+    branches:
+      - main
+      - staging
+  pull_request:
+    branches:
+      - main
 jobs:
   build:
     timeout-minutes: 5

.github/workflows/e2e.yml

@@ -1,5 +1,12 @@
 name: E2E test
-on: [push]
+on:
+  push:
+    branches:
+      - main
+      - staging
+  pull_request:
+    branches:
+      - main
 jobs:
   test:
     timeout-minutes: 10

.github/workflows/lint.yml

@@ -1,5 +1,8 @@
 name: Lint
-on: [push]
+on:
+  pull_request:
+    branches:
+      - main
 jobs:
   lint:
     runs-on: ubuntu-latest