add relative prefix to build script command

[breezykermo]

Running Node 8.9.3 and Rust 1.24.1.

When running npm install from xray_electron as suggested in contributing.md, the napi.js script triggered during the Cargo compilation of xray_node didn't look for target at the outside layer, where the code is built, but for a target folder inside the xray_node folder. Printing pwd from the child process cp confirms that it is executing inside the xray_node folder.

Adding this relative path fixed the issue for me. I'm not sure why { stdio: 'inherit' } doesn't work in the same way between lines 53 and 54. (Am new to rust and Cargo, is it something there?)

I also added two break statements to the other cases for execution clarity in the switch statement.

[breezykermo]

Note: this actually doesn't work, as in the case of building the nested napi/test-module, I need another .. to step out appropriately. Should the absolute path of the target directory be specified as a command line argument?

Did I miss some configuration step to appropriately locate the target folder?

[as-cii]

Hello @breezykermo, sorry we let this pull request sit around for 9 days and thanks for this contribution!

It looks like this issue may have been fixed already on master thanks to commit 5ffc386, so I am going to close it for now. If you still can reproduce the problem, please feel free to open a new issue and describe what you are observing. Thanks!