generate sass-variables.js before the build so it will be included in the dist output

building in postBuild doesn't work for CI:
- run ember test
- builds project to dist
- generates utils/sass-variables.js
- runs tests with code from dist, but utils/sass-variables.js not in that build because it didn't exist yet

it works for development (with `ember server`) because touching the file causes a rebuild
it works for production if a previous build (`ember test`) created the file