Fix #5776: CLI switch "--scripts-prepend-node-path" doesn't work

[brainfish]

Summary

This is a fix for #5776 where the --scripts-prepend-node-path CLI switch is ignored. The code to wire this flag up in src/config.js was missing.

Test plan

Before:

$ yarn run --scripts-prepend-node-path env | egrep 'NODE|PATH'
  "NODE": "/usr/local/Cellar/node/8.1.0/bin/node",
  "PATH": "/Users/brainfish/code/yarn/node_modules/.bin:...

After:

$ yarn run --scripts-prepend-node-path env | egrep 'NODE|PATH'
  "NODE": "/usr/local/Cellar/node/8.1.0/bin/node",
  "PATH": "/usr/local/Cellar/node/8.1.0/bin:/Users/brainfish/code/yarn/node_modules/.bin:...

[arcanis]

Thanks! 👍