chore: add scss:build step to yarn fastpass

[dbjorge]

Description of changes

The addition of yarn null:check to yarn fastpass means that fastpass now relies on yarn scss:build having happened first; since that step is relatively quick to execute anyway, this PR adds it to yarn fastpass to avoid users having to remember to do it manually.

Pull request checklist

• [n/a] Addresses an existing issue: #0000
• Ran yarn fastpass
• [n/a] Added/updated relevant unit test(s) (and ran yarn test)
• [n/a] Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS