build sass-variables.js in preBuild, not postBuild

[kielni]

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