Builds should fail when using MicroBuild test signing on mac & linux

When `_SignType=test` is specified in a macOS or Linux build, the MicroBuild plugin is intentionally skipped during installation since test signing is not supported on these platforms. However, this leads to build issues later in the process, and the root cause—test signing incompatibility with macOS and Linux—is not immediately clear.

To improve clarity, the build should fail early with a clear message indicating that test signing is not supported on macOS and Linux