This extends the calling convention for nint in a non-breaking way: If
the called script returns an attribute set instead of a string the
following is done:
* If the attributes `stdout` and/or `stderr` exist, their content (which
must be a string currently) is written to the respective output.
* If the attribute `exit` exists, nint will exit with the given exit
code. Must be a number that can be converted to an `i32`. If it's
missing, nint will exit without indicating an error.
Change-Id: I209cf178fee3d970fdea3b26e4049e944af47457
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3547
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>