MPP-4203 Free Onboarding Mobile Flow Missing 'Finish' Button

[vpremamozilla]

Fixes MPP-4203

How to test:

• Navigate to the Relay Landing page;
• Sign up with the non-Mozilla account;
• In the Free user onboarding, go to the third step (“Generate new mask” on the first step, “Next >” on the second step);
• Observe the contents of the step --> Finish button should now be showing

Screenshots

Checklist

• l10n changes have been submitted to the l10n repository, if any.
• I've added a unit test to test for potential regressions of this bug.
• I've added or updated relevant docs in the docs/ directory.
• All UI revisions follow the coding standards, and use Protocol / Nebula colors where applicable (see /frontend/src/styles/colors.scss).
• Commits in this PR are minimal and have descriptive commit messages.