add scripts in builder-tools/* into the PATH
add scripts in builder-tools/* into the PATH

Before, we attempted to add these scripts into the PATH by simply
modifying the path to reflect /workspace/... within the Docker image's
filesystem. However this is a problem in Google Cloud Build because
/workspace becomes "empty" at the start of a build.

The solution is to just copy over the scripts into the top-level
/k8s-addon-builder folder, as they were done previously (prior to
665362dad2a260b05dcdb66a6ed3d7a5da9eab07).

GoogleCloudPlatform/k8s-addon-builder,GoogleCloudPlatform/k8s-addon-builder,GoogleCloudPlatform/k8s-addon-builder