Refactor drone CI integration

The readme says:

> Copy the .drone.star file and .drone directory from this repository to the top level of your repository.

So what will happen is that e.g. `linux_cxx` will Pull down Boost.CI: https://github.com/boostorg/boost-ci/blob/4ed2aa3e9c7b21c7b5e8dd3f437dd9e3725b7667/ci/drone/functions.star#L61

And the current example drone file will do that again: https://github.com/boostorg/boost-ci/blob/4ed2aa3e9c7b21c7b5e8dd3f437dd9e3725b7667/.drone/drone.sh#L21

So this duplicates work wasting resources.

Furthermore the [Drone config](https://github.com/boostorg/boost-ci/blob/4ed2aa3e9c7b21c7b5e8dd3f437dd9e3725b7667/.drone.star#L17-L31) is much less readable than e.g. the [Github config](https://github.com/boostorg/boost-ci/blob/4ed2aa3e9c7b21c7b5e8dd3f437dd9e3725b7667/.github/workflows/ci.yml#L46-L62)

@sdarwin Could you work on the first part, i.e. that it downloads Boost.CI at most once and only if not running on Boost.CI? I don't understand why we would need `.drone/boost-ci` and `ci` when having the latter is enough (see e.g. Github actions)

I started work on an improved drone config in a [branch](https://github.com/boostorg/boost-ci/tree/drone-refactor)