Restrict permissions in GitHub workflows (#10790)

.github/workflows/main.yml

@@ -2,6 +2,9 @@ name: CI
 
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   ubuntu:
     runs-on: ubuntu-18.04

.github/workflows/nodejs.yml

@@ -2,6 +2,9 @@ name: CI (node.js)
 
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: ubuntu-latest