Download elastic-agent once

[mdelapenya]

We'd like to download it from the test runner instead of doing it inside the container, then we would copy it to the running container when deploying the elastic-agent to fleet.

This will add two things:

1. reliability, as we will download the file once, and possibly using retries, which is not possible (or more difficult) to do it in the container
2. reduce build time: it takes around 7 seconds to download the file in my local machine. Simply counting the times the Deploy agent step is used (6 times for fleet at this moment) will give us the build time reduction.