Update workflow practices

Author: jwodder

.github/workflows/validate.yml

@@ -3,13 +3,23 @@ name: Validate
 on:
   pull_request:
   push:
+    branches:
+      - master
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.ref_name }}
+  cancel-in-progress: true
+
+permissions: {}
 
 jobs:
   test:
     runs-on: ubuntu-latest
     steps:
       - name: Check out repository
         uses: actions/checkout@v6
+        with:
+          persist-credentials: false
 
       - name: Set up Python
         uses: actions/setup-python@v6