This adds an additional parameter `entrypoint`, pointing to a .nix file
(or a directory containing a `default.nix` file) that's providing the
attribute path asked for.
If not set / kept at the default (empty string), it falls back to the
root dir of the repository as before.
Change-Id: I2e63114f21660c842153ac15424b3491d66624d2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8190
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>