ci: drop the write permissions

They were added for the step that commented on the pull request. The
sizes go to the job summary now, which only needs to read the
repository.

.github/workflows/build.yml

@@ -7,8 +7,6 @@ on:
 
 permissions:
   contents: read
-  pull-requests: write
-  issues: write
 
 jobs:
   build: