loader: Fetch initial script from loader_lua env

Sometimes it is nice to override the initial script that we run. Make it
possible by fetching loader_lua from the env and using that instead of
the default if prsent.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43819

(cherry picked from commit cd147a2a024301a796f307c7bae686305d2bf302)