ci: correct perms for tag-service.yml (ethereum-optimism#11961)

GITHUB_TOKEN no longer gets write access by default so it needs to
be provided with write access explicitly.

.github/workflows/tag-service.yml

@@ -36,6 +36,9 @@ on:
         type: boolean
         default: false
 
+permissions:
+  contents: write
+
 jobs:
   release:
     runs-on: ubuntu-latest