fix: remove root-owned overlay path from buildah cache

The /var/lib/containers/storage/overlay directory is owned by root
(from the container-storage-action BTRFS mount). The actions/cache
post-cleanup step cannot remove it as the runner user, causing EACCES
warnings.

Co-Authored-By: opencode <opencode@opencode.ai>

.github/workflows/build.yml

@@ -84,7 +84,6 @@ jobs:
         uses: actions/cache@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
         with:
           path: |
-            /var/lib/containers/storage/overlay
             ~/.cache/buildah
             /tmp/.buildah-cache
           key: ${{ runner.os }}-${{ github.workflow }}-${{ hashFiles('Containerfile', 'build/**', 'configs/**', 'scripts/**') }}-${{ github.ref }}