Make Starlark file size limit configurable

[andrii-kasparevych]

This PR introduces a way to configure the Starlark generated YAML size limit as opposed to having a hard-coded value.
The default behavior does not change, but a user can set DRONE_STARLARK_SIZE_LIMIT env var on the drone server to allow for a larger file.
This would resolve an issue described here.

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[bradrydzewski]

Thanks @andrii-kasparevych for the pull request. @eoinmcafee00 could you please take a look? I did a quick glance and it looks good, but at this point you know the code better than I do :)

[eoinmcafee00]

hi @andrii-kasparevych

Thanks again for the submission, it now has been merged.