[CI] Don't trigger GHA pull_request on PRs from local branches

.github/workflows/aarch64.yml

@@ -1,6 +1,14 @@
 name: AArch64 CI
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+    branches:
+      # Branches from forks have the form 'user:branch-name' so we only run
+      # this job on pull_request events for branches that look like fork
+      # branches. Without this we would end up running this job twice for non
+      # forked PRs, once for the push and then once for opening the PR.
+      - "**:**"
 
 jobs:
   musl-build:

.github/workflows/linux.yml

@@ -1,6 +1,14 @@
 name: Linux CI
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+    branches:
+      # Branches from forks have the form 'user:branch-name' so we only run
+      # this job on pull_request events for branches that look like fork
+      # branches. Without this we would end up running this job twice for non
+      # forked PRs, once for the push and then once for opening the PR.
+      - "**:**"
 
 env:
   TRAVIS_OS_NAME: linux

.github/workflows/macos.yml

@@ -1,6 +1,14 @@
 name: macOS CI
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+    branches:
+      # Branches from forks have the form 'user:branch-name' so we only run
+      # this job on pull_request events for branches that look like fork
+      # branches. Without this we would end up running this job twice for non
+      # forked PRs, once for the push and then once for opening the PR.
+      - "**:**"
 
 env:
   SPEC_SPLIT_DOTS: 160

.github/workflows/win.yml

@@ -1,6 +1,14 @@
 name: Windows CI
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+    branches:
+      # Branches from forks have the form 'user:branch-name' so we only run
+      # this job on pull_request events for branches that look like fork
+      # branches. Without this we would end up running this job twice for non
+      # forked PRs, once for the push and then once for opening the PR.
+      - "**:**"
 
 jobs:
   linux-job: