fix: npm output handling

[sriram-mv]

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]

Couldnt push to #112