Fix edge cases with vendoring

I encountered two edge cases that threw exceptions when running "berks vendor" with berks 3.2.1:
* the PATH provided to "berks vendor" is relative
* a cookbook being vendored contains read-only files (such as mycookbook/.git/objects/13/310f74cd58123013ffdf6bd616456793ad44fb)

@rchekaluk specifically [around this line](https://github.com/berkshelf/berkshelf/blob/master/lib/berkshelf/berksfile.rb#L617).