Refine workflow push trigger (#1182)

Summary:
We run validation workflows on push, but we do this for every branch, so dependabot PRs run every validation twice. We only really want push validation for the main branch.

Pull Request resolved: https://github.com/facebook/yoga/pull/1182

Test Plan: PRs still run

Reviewed By: cortinico

Differential Revision: D41741459

Pulled By: NickGerleman

fbshipit-source-id: 51abe2cc0c8c5b9c3fc8c8a20a585c9d5e868a5e

.github/workflows/validate-android.yml

@@ -1,6 +1,11 @@
 name: Android
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
 
 jobs:
   build:

.github/workflows/validate-apple.yml

@@ -1,6 +1,11 @@
 name: Apple
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
 
 jobs:
   lint-pods:

.github/workflows/validate-cpp.yml

@@ -1,6 +1,11 @@
 name: C++
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
 
 jobs:
   clang-format:

.github/workflows/validate-website.yml

@@ -1,6 +1,11 @@
 name: Website
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
 
 jobs:
   build: