Ninja builder cleanup

We're in the process of switching to the latest gftools using the Ninja-based builder. However, when I build a simple `.ufo` family, the build leaves behind an `instance_ttf` directory in the root of the directory and a `.designspace` file alongside the UFO sources.

My expectation is that temporary files are removed after the build and the source directory is not modified. Is this an intentional change?

Reproduce the build for a simple .ufo family with the Ninja-based builder and inspect the generated instance_ttf directory and .designspace file. Trace where these outputs are created or cleaned up, then verify that temporary files are removed and the source directory remains unchanged after the build.

Written by the indexing model from the issue text.