Actions: Run CI for PRs only from forks.

.github/workflows/black.yaml

@@ -4,6 +4,10 @@ on: ["push", "pull_request"]
 
 jobs:
   black:
+    # We want to run on external PRs, but not on our own internal PRs as they'll be run
+    # by the push to the branch.
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+
     runs-on: ubuntu-latest
     steps:
       - name: Setup Python

.github/workflows/linux.yml

@@ -6,6 +6,10 @@ on:
 
 jobs:
   build:
+    # We want to run on external PRs, but not on our own internal PRs as they'll be run
+    # by the push to the branch.
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+
     runs-on: ubuntu-latest
     steps:
       - name: Checkout

.github/workflows/windows.yaml

@@ -1,14 +1,15 @@
 name: Build and test windows wheels
 on:
   push:
-    branches:
-      - master
-      - test
   pull_request:
   create:
 
 jobs:
   build:
+    # We want to run on external PRs, but not on our own internal PRs as they'll be run
+    # by the push to the branch.
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+
     runs-on: windows-latest
     steps:
       - name: Checkout