Automerge: [libc++][Github] Remove workflow-scoped write permissions (#126447)

This patch removes the workflow-scoped package write permissions in the
libcxx-build-containers workflow. The relevant permissions are already
present in the job, so this raises the potential for new jobs being
added to the workflow that do not need the permissions but having them
anyways. Not having workflow-scoped write permissions is security best
practice.

Fixes #126230.

Author: boomanaiden154