Fix beats magefiles breaking changes

[moukoublen]

Currently there are some breaking changes when updating beats dependency.

Some of those (libbeat) are resolved in this PR but there seems to be a new way of producing the field docs. This specific change makes the field doc generation command not work for Cloudbeat because when Cloudbeat runs the mage files run from inside the dependency folder, so the path changes.

We might need to update the beats relevant code to something overridable from the cloudbeat side.