build-sass: Support optional out-dir option

[aduth]

🛠 Summary of changes

Improves @18f/identity-build-sass to allow usage without specifying --out-dir flag.

I stumbled upon this when trying to use the package outside the project for reproduction steps for an unrelated bug report.

📜 Testing Plan

git checkout aduth-build-sass-optional-out-dir
touch example.scss
yarn build-sass example.scss
ls example.css

Observe that output file exists.