Fixed distribution archive format.

[cmaglie]

Previously, the zip archives were created without a root folder. This PR fixes this issue.
This is actually a regression after #93, which removed the DistTask section:

-mkdir -p {{.PLATFORM_DIR}}
-cp ../LICENSE.txt {{.PLATFORM_DIR}}
 docker run ...

You must run the mkdir command before starting Docker to ensure write permission on the PLATFORM_DIR directory; otherwise, the directory will be created by the go build command inside the Docker container, which runs as root, making it impossible to copy the LICENSE file later.

We might want to push this change upstream in the assets repo. /cc @per1234