Bug 1071312 - [FTU][FxA] Button still reads as 'Skip' after the user log...

...s into a Firefox Account in the FTE.
either we use the same way of updating translations that we are using in the rest of the file:
```
navigator.mozL10n.localize(
  nextButton.firstChild,
  'navbar-next'
);
```
or the new way, which is slowly being updated (it should be done in bug 1058985 anyways):
```
nextButton.firstChild.setAttribute('data-l10n-id', 'navbar-next');
```