github: specify workflow permissions

Update gh-pages has started failing with default permissions. It needs
write permissions to be able to push to the gh-pages branch.

(cherry picked from commit b024fa1)

Author: marquiz

.github/workflows/gh-pages.yml

@@ -6,10 +6,19 @@ on:
       - release-*
     tags:
       - v[0-9]+.[0-9]+.[0-9]+
+
+concurrency:
+  group: gh-pages
+
+permissions:
+  contents: read
+
 jobs:
   build:
     name: Update gh-pages
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
     steps:
 
     - name: Check out repo