fix output handling

[abetaev]

Hi,

Issue
If prepack script is defined npm seems to output more than expected in original code.

Description of changes:
npm pack outputs filename as last line of stdout, while in code the whole its output was assigned to tarfile_name var. This fix makes it right.

Thanks,
Alex

p.s. By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sriram-mv]

Does this change work in either cases? if prepack is defined or not defined?

[mhart]

Lol, just ran into this – and fixed with it the same fix. Was just about to submit a PR.

It also happens if you have a prepare script defined.

This fix is the right fix – npm can output a bunch of stuff to stdout if it runs user scripts during the pack process, and aws-lambda-builders really messes up the tar file name in that case (and errors).