clean the current Fuchsia build artifacts

    Delete the contents of the output directory except for args.gn and
    create a Ninja build environment sufficient to regenerate the build.

    This is equivalent to running `gn clean` on FUCHSIA_BUILD_DIR, then
    invoking `fx gen` to regenerate critical build files.

[clean source code](https://cs.opensource.google/fuchsia/fuchsia/+/main:tools/devshell/clean)