MicroBuild Plugin Causes Automatic Signing Without `--sign`

When the MicroBuild plugin is installed and a project containing `FilesToSign` and a MicroBuild package reference is built, MicroBuild signing will run — **even if the `--sign` flag was not passed**.

This leads to unintended behavior: files may be signed automatically without an explicit request to do so, which can result in accidental signing.

When a repo is built via arcade, files should only be signed when explicitly requested — such as by passing the `--sign` flag.