[node-build-scripts] sass-lint path logic is broken

Environment

• Package version(s): node-build-scripts v1.6.x

Steps to reproduce

1. Use sass-lint.js script outside of this repo, through @blueprintjs/node-build-scripts

Actual behavior

It always fails to find .stylelintrc because it's looking inside the node_modules folder where the scripts package is installed.

Expected behavior

The script is usable outside this repo...

Possible solution

Use process.cwd() instead of __dirname, and also allow a CLI argument to override the configFile option