Resolve release-plz failure from publish mismatch

Cargo.toml has `publish = false`, release-plz expects the same in its
config file.

Author: tgross35

.release-plz.toml

@@ -3,6 +3,7 @@ name = "libc"
 changelog_path = "CHANGELOG.md"
 git_release_name = "{{ version }}"
 git_tag_name = "{{ version }}"
+publish = false # On the main branch, we don't want to publish anything
 
 [[package]]
 name = "ctest"